Methods of using ozone to degrade organic material

ABSTRACT

Methods of using ozone have been developed which sterilize instruments and medical wastes, oxidize organics found in wastewater, clean laundry, break down contaminants in soil into a form more readily digested by microbes, kill microorganisms present in food products, and destroy toxins present in food products. The preferred methods for killing microorganisms and destroying toxins use pressurized, humidified, and concentrated ozone produced by an electrochemical cell.

[0001] This application is a continuation-in-part application of pendingU.S. patent application Ser. No. 08/091,752, filed Jul. 13, 1993.

[0002] It is to be noted that the U.S. government may have rights in thesubject matter disclosed and claimed herein.

BACKGROUND OF THE INVENTION

[0003] This invention relates generally to improvements in theproduction of ozone (O₃). More particularly, the invention relates tothe electrolytic production of ozone utilizing a proton exchangemembrane to separate the anode and depolarized cathode.

[0004] Ozone has long been recognized as a useful chemical commodityvalued particularly for its outstanding oxidative activity. Because ofthis activity it finds wide application in disinfection processes. Infact, it kills bacteria more rapidly than chlorine, it decomposesorganic molecules, and removes coloration in aqueous systems. Ozonationremoves cyanides, phenols, iron, maganese, and detergents. It controlsslime formation in aqueous systems, yet maintains a high oxygen contentin the system. Unlike chlorination, which may leave undesirablechlorinated organic residues in organic containing systems, ozonationleaves fewer potentially harmful residues. There is evidence that ozonewill destroy viruses. It is used for sterilization in the brewingindustry and for odor control in sewage treatment and manufacturing. Andozone is employed as a raw material in the manufacture of certainorganic compounds, e.g., oleic acid and peroxyacetic acid.

[0005] Thus, ozone has widespread application in many diverseactivities, and its use would undoubtedly expand if its cost ofproduction could be reduced. In addition, since ozone is explosive whenconcentrated as either a gas or liquid, or when dissolved into solventsor absorbed into gels, its transportation is potentially hazardous.Therefore, it is generally manufactured on the site where it is used.However, the cost of generating equipment, and poor energy efficiency ofproduction has deterred its use in many applications and in manylocations.

[0006] On a commercial basis, ozone is currently produced by the silentelectric discharge process, otherwise known as corona discharge, whereinair or oxygen is passed through an intense, high frequency alternatingcurrent electric field. The corona discharge process forms ozone throughthe following reaction:

3/20₂=O_(3;)ΔH°₂₉₈=34.1 kcal

[0007] Yields in the corona discharge process generally are in thevicinity of 2% ozone, i.e., the exit gas may be about 2% O₃ by weight.Such O₃ concentrations, while quite poor, in an absolute sense, arestill sufficiently high to furnish usable quantities of O₃ for theindicated commercial purposes. Another disadvantage of the coronaprocess is the production of harmful NO_(x), otherwise known as nitrogenoxides. Other than the aforementioned electric discharge process, thereis no other commercially exploited process for producing largequantities of O₃.

[0008] However, O₃ may also be produced by the electrolytic process,wherein an electric current (normally D.C.) is impressed acrosselectrodes immersed in an electrolyte, i.e., electrically conducting,fluid. The electrolyte includes water, which, in the process,dissociates into its respective elemental species, O₂ and H2. Under theproper conditions, the oxygen is also evolved as the O₃ species. Theevolution of O₃ may be represented as:

3H₂O=O₃+3H_(2;)ΔH°₂₉₈=207.5 kcal

[0009] It will be noted that the DH° in the electrolytic process is manytimes greater than that for the electric discharge process. Thus, theelectrolytic process appears to be at about a six-fold disadvantage.

[0010] More specifically, to compete on an energy cost basis with theelectric discharge method, an electrolytic process must yield at least asix-fold increase in ozone. Heretofore, the necessary high yields havenot been realized in any foreseeably practical electrolytic system.

[0011] The evolution of O₃ by electrolysis of various electrolytes hasbeen known for well over 100 years. High yields up to 35% currentefficiency have been noted in the literature. Current efficiency is ameasure of ozone production relative to oxygen production for giveninputs of electrical current, i.e., 35% current efficiency means thatunder the conditions stated, the O₂/O₃ gases evolved at the anode arecomprised of 35% O₃ by volume. However, such yields could only beachieved utilizing very low electrolyte temperatures, e.g., in the rangefrom about −30° C. to about −65° C. Maintaining the necessary lowtemperatures, obviously requires costly refrigeration equipment as wellas the attendant additional energy cost of operation.

[0012] Ozone, O₃, is present in large quantities in the upper atmospherein the earth to protect the earth from the suns harmful ultravioletrays. In addition, ozone has been used in various chemical processes, isknown to be a strong oxidant, having an oxidation potential of 2.07volts. This potential makes it the fourth strongest oxidizing chemicalknown.

[0013] Because ozone has such a strong oxidation potential, it has avery short half-life. For example, ozone which has been solubilized inwaste water may decompose in a matter of 20 minutes. Ozone can decomposeinto secondary oxidants such as highly reactive hydroxyl (OH*) andperoxyl (HO₂*) radicals. These radicals are among the most reactiveoxidizing species known. They undergo fast, non selective, free radicalreactions with dissolved compounds. Hydroxyl radicals have an oxidationpotential of 2.8 volts (V), which is higher than most chemical oxidizingspecies including O₃. Most of the OH* radicals are produced in chainreactions where OH itself or HO₂* act as initiators.

[0014] Hydroxyl radicals act on organic contaminants either by hydrogenabstraction or by hydrogen addition to a double bond, the resultingradicals disproportionate or combine with each other forming many typesof intermediates which react further to produce peroxides, aldehydes andhydrogen peroxide.

[0015] Electrochemical cells in which a chemical reaction is forced byadded electrical energy are called electrolytic cells. Central to theoperation of any cell is the occurrence of oxidation and reductionreactions which produce or consume electrons. These reactions take placeat electrode/solution interfaces, where the electrodes must be goodelectronic conductors. In operation, a cell is connected to an externalload or to an external voltage source, and electric charge istransferred by electrons between the anode and the cathode through theexternal circuit. To complete the electric circuit through the cell, anadditional mechanism must exist for internal charge transfer. This isprovided by one or more electrolytes, which support charge transfer byionic conduction. Electrolytes must be poor electronic conductors toprevent internal short circuiting of the cell.

[0016] The simplest electrochemical cell consists of at least twoelectrodes and one or more electrolytes. The electrode at which theelectron producing oxidation reaction occurs is the anode. The electrodeat which an electron consuming reduction reaction occurs is called thecathode. The direction of the electron flow in the external circuit isalways from anode to cathode.

[0017] A typical electrochemical cell will have a positively chargedanode and a negatively charged cathode. The anode and cathode aretypically submerged in a liquid electrolytic solution which may becomprised of water and certain salts, acids or base materials. Generallyspeaking, gaseous oxygen is released at the anode surface while gaseoushydrogen is released at the cathode surface. A catalyst such as leaddioxide may be used to coat the anode to get greater ozone production.The anode substrate may be another material such as titanium, graphite,or the like.

[0018] The cathode and anode are positioned within the electrolytic cellwith electrical leads leading to the exterior. The cell is also providedwith appropriate plumbing and external structures to permit circulationof the electrolyte to a separate heat exchanger. Suitable inlet andoutlet passages are also provided in the cell head space to permit thewithdrawal of the gases evolved from the cathode (if gases are to beevolved) and from the anode. The two gas removal systems are typicallymaintained separate in order to isolate the cathode gases from the anodegases. Nitrogen and/or air may be pumped through the gas handling systemin order to entrain the evolved cathode and anode gases and carry themfrom the cell to the exterior where they may be utilized in the desiredapplication. Alternately, if a flow-through air or oxygen cathode isemployed, its excess gases may be used for this purpose.

[0019] In order to maintain or cool the cell electrodes, heat exchangepassages may be provided within the electrode structures. These coolantpassages are connected to external sources of coolant liquid which canbe circulated through the electrodes during the electrolysis process inorder to maintain or reduce their temperatures.

[0020] In order to drive the electrolysis reaction, it is necessary toapply electric power to the cell electrodes. The electrodes areconnected through the electrical leads to an external source of electricpower with the polarity being selected to induce the electrolyte anionflow to the anode and the cation flow to the cathode. The powerrequirements are not appreciably different for those cells utilizingplatinum anodes from those cells utilizing lead dioxide anodes.Electrical potentials on the order of from 2-3 volts D.C. are quitesufficient for the various cell configurations. The current requirementsare most easily measured in terms of current density and may vary from alow of perhaps a tenth of an ampere per square centimeter (0.1 A/cm²) upto current densities slightly beyond one ampere per square centimeter(>1.0 A/cm²). The power requirements are not necessarily dependent uponthe electrolyte concentrations, nor in particular upon the anodematerials. Thus, current densities of from about 0.1 A/cm² to about 1.5A/cm² will produce maximum ozone current efficiencies at any electrolyteconcentration with either beta lead dioxide anodes or platinum anodes.

[0021] U.S. Pat. No. 4,316,782 (Foller) teaches that ozone yields ashigh as 52% could be obtained where the electrolyte is water and eitherthe acid or salt form of highly electronegative anions, such ashexafluoro-anions are used. Here, the term “fluoro-anions” is used todescribe that family of anionic (negatively charged) species in whichmultiple fluorine ligands complex a central atom. Electrolysis wascarried out in a range between room temperature and the freezing pointof water. The preferred anode materials for use in the electrolyticcells are either platinum or lead dioxide, especially lead dioxide inthe beta crystalline form. Platinum, carbon, or nickel and its alloysmay be used as hydrogen-evolving cathodes. Alternatively, an air oroxygen depolarized cathode may be employed which would greatly reducethe cell voltage and enhance the overall energy efficiency of theprocess.

[0022] Such electrolytic solutions can be highly corrosive to the cellmaterials if they are not selected properly, and especially hard on theelectrodes where electrochemical discharge takes place. In addition, theliberated O₃, being a powerful oxidizing agent, also strongly acts uponelectrode materials which are susceptible to oxidizing action. Theelectrical properties of the electrode material are also important tothe successful and effective operation of the ozone generatingelectrolytic cell. The electrodes must exhibit sufficient electricalconductivity to enable the utilization of current densities required bythe ozone generating process without an unacceptable anode potential andmust also be adaptable to whatever cooling procedures are required tomaintain cell temperatures during operation.

[0023] Foller also disclosed that using an air or oxygen depolarizedcathode provided several advantages. (1) The cell voltage would besubstantially reduced since replacing hydrogen evolution with thereduction of oxygen theoretically saves 1.23 volts. (In actual practicea 0.8 volt swing is likely to be achieved.) (2) A separator betweenanode and cathode is no longer required, as no hydrogen is evolved todepolarize the anode. Further, savings in cell voltage are obtained byreducing IR losses. (3) The overall cell process becomes oxygen in andozone out and the need for periodic additions of water is reduced. (4)The same air or oxygen fed to the air cathode could also serve to diluteand carry off the ozone that is anodically evolved by flowing throughthe cathode.

[0024] Air cathode technology has found recent favor in its applicationto fuel cells, metal-air batteries, and the chlor-alkali industry. Theelectrodes are generally composed of Teflon-bonded carbon containingsmall amounts of catalytic materials.

[0025] U.S. Pat. No. 4,375,395 (Foller) teaches that anodes made ofglassy carbon are suitable for use in the preparation of ozone in anelectrolytic cell utilizing an aqueous solution of the highlyelectronegative fluoro-anions.

[0026] U.S. Pat. No. 4,541,989 (Foller) teaches that a liquidelectrolyte containing acids of fluoro-anions, such as HBF₄ and HPF₆,used in combination with a cool electrolyte solution can increase theefficiency and the ozone to oxygen yield. However, the use of a liquidelectrolyte causes some problems. First, the electrodes in suchelectrolytic cell must be separated by a given distance to providedefinition. This translates into power loss in the production of heat.Secondly, the presence of liquid electrolytes requires a sophisticatedsystem of seals to prevent leaking of the electrolyte.

[0027] U.S. Pat. No. 4,836,929 (Laumann et al.) teaches the use of asolid electrolyte such as that made by duPont and sold under the brand“NAFION”. This solid electrolyte was placed between a lead dioxide anodeand a platinum black cathode. The current efficiency was increased byoxygenating a water stream fed to the anode and the cathode. In thismanner, oxygen could be reduced to water at room temperature releasingan increased yield of ozone.

[0028] In his paper entitled “Synthesis of Hydrogen Peroxide in a ProtonExchange Membrane Electrochemical Reactor” (Apr. 1993), Fenton disclosedthat paired synthesis of ozone (O₃) and hydrogen peroxide (H₂O₂) couldbe carried out in the same reactor. The electrochemical reactor used amembrane and electrode assembly (M&E) comprised of a “NAFION” 117membrane between the platinum black/polytetrafluoroethylene (PTFE) anodeand graphite/PTFE cathode. This M&E assembly was sandwiched between acarbon fiber paper (Toray Industries) on the cathode side and a platinummesh (52 mesh, Fisher Scientific) on the anode side which were used ascurrent collectors. This arrangement was alleged to produce somehydrogen peroxide.

[0029] Increasing the percentage of PTFE in the electrode increases thehydrophobicity of the electrode assembly and thus allows more of thegaseous reactant to reach the electrode surface by repelling theproducts formed. The graphite M&E with 20% PTFE produced slightly higherhydrogen peroxide than a similar M&E with 10% PTFE. This could be due tothe mass transport limitation of oxygen to the membrane and electrodeassembly within the less hydrophobic 10% M&E. It is preferred that thePEM reactor operate at potentials greater than 3.0 volts where theanodic evolution of ozone is favored.

[0030] Membranes containing perfluorinated sulphonic acids are typicallyprepared before use in an electrochemical cell by first soaking themembrane in hot water for about 30 minutes and then soaking it in 10%HCl to ensure that the entire membrane is in the H⁺form. The membranehas to be kept moist at all times as it acts as a conductor only when itis wet. It is preferred that the proton exchange membrane be pretreatedwith an aqueous solution of sulphuric acid followed by rinsing theproton exchange membrane with pure water, rinsing the proton exchangemembrane with an aqueous solution of hydrogen peroxide, and rinsing theproton exchange membrane with a final rinse of pure water. The finalrinse should be made at a temperature between 50° C. and 150° C. andunder pressure.

[0031] In their paper entitled “Paired Synthesis of Ozone and HydrogenPeroxide in an Electrochemical Reactor,” Pallav Tatapudi and JamesFenton explain that the benefits of paired synthesis in electrolyte freewater include: (1) lower energy consumption costs, as two oxidizingagents can be obtained for the price of one; (2) the elimination of theneed for transportation and storage of oxidants by generating themelectrochemically within water on demand at an amount proportional tothe waste concentration; and (3) higher aqueous phase ozoneconcentrations.

[0032] U.S. Pat. No. 4,416,747 (Menth et al.) discloses an individualelectrolysis cell bounded by bipolar plates and having a solidelectrolyte made of perfluorinated sulphonic acids (“NAFION” by duPont)with a surface coating centrally located between current-collectors andadjoining open metallic structures. A plurality of individual cells maybe integrated together between end plates so that the cells areelectrically connected in series, hydrodynamically connected inparallel, and combined to form a block.

[0033] The current collectors disclosed in Menth may be close-meshedexpanded metal covered by an open structure having a low resistance tothe flow of a liquid in the direction parallel to the planar structure.The current collectors are preferably made from titanium. The ends ofthe cell are formed, in each case, by a bipolar plate, which alternatelyacts as a cathode and as an anode. The bipolar plate is preferably madefrom stainless (Cr/Ni) steel. The space or chamber between the bipolarplates and the solid electrolyte is completely filled with water inwhich air or oxygen is suspended and/or dissolved.

[0034] The Menth assembly of the electrolysis cells basicallycorresponds to the filter-press type, with the liquid passing parallelto the principal planes of the cells instead of perpendicularly. Theindividual cells are held together between two end plates havingelectrical terminals thereon.

[0035] The method and apparatus disclosed in Menth, however, can supportonly limited current density associated with reduction-oxidation sinceoxygen has only limited solubility in water. Further, since the cathodechamber is filled with liquid water, the cathode electrode structurewill become flooded with water. Higher current densities are desirableto cause an increase in the ozone production efficiency.

[0036] U.S. Pat. No. 4,836,849 (Laumann et al.) teaches a process forbreaking down organic substances and/or microbes in pretreated feedwater for high-purity recirculation systems using ozone which isgenerated in the anode chamber of an electrochemical cell and treatedwith ultraviolet rays and/or with hydrogen generated in the cathodechamber of the same cell or supplied from outside.

[0037] In light of the foregoing discussion, there exists a need for aneconomical method of producing ozone which will minimize voltage allowedfor higher current density and produce a high concentration of ozone.

SUMMARY OF THE INVENTION

[0038] The present invention is a method for electrochemical synthesisof ozone. A source of a cathodic depolarizer is supplied to a cathodedisposed in a cathodic chamber and water is supplied to an anodedisposed in an anodic chamber. Electricity is then passed through anionically conducting electrolyte that is disposed in the anodic andcathodic chambers such that the electrolyte is in intimate contact withboth the anode and the cathode. The cathodic depolarizer is reduced atthe cathode and the water is oxidized to ozone at the anode. The ozonegas produced at the anode is then withdrawn from the anodic chamber.

[0039] An apparatus for the electrolytic generation of ozone maycomprise an anode, gas diffusion cathode and proton exchange membrane.The anode comprises a substrate and a catalyst coating wherein thesubstrate is selected from the group consisting of porous titanium,titanium suboxides (such as that produced by Atraverda Limited under thetrademark “EBONEX”), platinum, tungsten, tantalum, hafnium and niobium,and wherein the catalyst coating is selected from the group consistingof lead dioxide, platinum-tungsten alloys or mixtures, glassy carbon andplatinum.

[0040] The gas diffusion cathode comprises apolytetrafluoroethylene-bonded, semi-hydrophobic catalyst layersupported on a hydrophobic gas diffusion layer. The catalyst layer iscomprised of a proton exchange polymer, polytetrafluorethylene polymerand a metal selected from the group consisting of platinum, palladium,gold, iridium, nickel and mixtures thereof. The gas diffusion layer hasa carbon cloth or carbon paper fiber impregnated with a sintered massderived from fine carbon powder and a polytetrafluoroethylene emulsion.

[0041] The ionically conducting electrolyte is typically a protonexchange membrane having a first side bonded to the catalyst layer ofthe gas diffusion cathode and a second side in contact with the anode.The preferred material for the proton exchange membrane is aperfluoronated sulfonic acid polymer.

[0042] An apparatus for the electrolytic generation of ozone maycomprise a plurality of individual electrolytic cells where each cellhas an anode, gas diffusion cathode and proton exchange membrane, asdescribed above. This multiple cell arrangement further includes firstand second electrically insulating, chemically resistant gasketsdisposed around the edges of the anode and cathode, respectively, havingsections removed for internal manifolding to allow fluid flow to andfrom the electrode/electrolyte interfaces. First and second expandedmetal communicates electrically between the electrode and an adjacentbipolar plate and facilitates fluid flow over the entire electrodesurface. Each of these individual electrolytic cells are positioned in afilter-press type arrangement and connected in a series electricalcircuit. The bipolar plate disposed between each of the individualelectrolytic cells has a first side in electrical contact with the anodeof a first adjacent cell and a second side in electrical contact withthe cathode of a second adjacent cell. The apparatus also includes a setof two end plates having electrical connection means, an oxygen gas orair inlet port, a water inlet port, a cathode product outlet port and ananode product outlet port. Clamping means are used to secure the endplates and electrolytic cells tightly together.

[0043] Ozone may be electrochemically produced by supplying a source ofoxygen gas to a gas diffusion cathode, wherein the gas diffusion cathodeincludes a gas diffusion layer and a catalyst layer. The gas diffusionlayer comprises carbon cloth or carbon paper fiber impregnated with asintered mass derived from fine carbon powder and apolytetrafluoroethylene emulsion. The catalyst layer comprises a protonexchange polymer, polytetrafluoroethylene polymer and a metal selectedfrom the group consisting of platinum, palladium, gold, iridium, nickeland mixtures thereof. Water is supplied to an anode comprising asubstrate and a catalyst coating. The anodic substrate is selected fromthe group consisting of porous titanium, titanium suboxides, platinum,tungsten, tantalum, hafnium and niobium. The anodic catalyst coating isselected from the group consisting of lead dioxide, platinum-tungstenalloys or mixtures, glassy carbon and platinum. Electric current is thenpassed through the anode and the gas diffusion cathode, which areseparated by a proton exchange membrane. The proton exchange membrane iscomprised of a perfluoronated sulfonic acid polymer material that isbonded to the catalyst layer of the gas diffusion cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] So that the manner in which the above recited features,advantages and objects of the present invention are attained and can beunderstood in detail, a more particular description of the invention,briefly summarized above, may be had by reference to the embodimentsthereof which are illustrated in the appended drawings.

[0045] It is to be noted, however, that the appended drawings illustrateonly typical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

[0046]FIG. 1 is a cross-sectional side view of a depolarized ozoneelectrolysis cell;

[0047]FIG. 2 is a cross-sectional side view of a second embodiment of adepolarized ozone electrolysis cell;

[0048]FIG. 3 is a chart showing the variation of cell voltage withcurrent density for the oxygen depolarized ozone electrolysis cell ofFIG. 2;

[0049]FIG. 4 is a chart showing the variation of ozone currentefficiency with current density for the oxygen depolarized ozoneelectrolysis cell of FIG. 2;

[0050]FIG. 5 is a cross-sectional view of a multiple cell arrangement ofthe depolarized ozone electrolysis cell of FIG. 2 where the cells areelectrically connected in a series circuit;

[0051]FIG. 6 is a schematic diagram of the apparatus of FIG. 5 incombination with ancillary equipment for the electrochemical productionof ozone gas and/or ozonated water;

[0052]FIG. 7 is a schematic diagram of the apparatus of FIG. 6 incombination with ancillary equipment used in a process for thepurification of water;

[0053]FIG. 8 is a chart showing the variation of cell voltage withcurrent density for the air depolarized ozone electrolysis cell of FIG.1;

[0054]FIG. 9 is a chart showing the variation of ozone currentefficiency with electrochemical reactor temperature for the airdepolarized ozone electrolysis cell of FIG. 1; and

[0055]FIG. 10 is a chart showing the variation of hydrogen peroxideproduction rate with current density for the air depolarized ozoneelectrolysis cell of FIG. 1.

[0056]FIG. 11 is a chart showing the variation of the percentage ofspore strips sterilized with the duration of ozone exposure for sporestrips enclosed in glassine envelopes;

[0057]FIG. 12 is a chart showing the variation of the percentage ofspore strips sterilized with the duration of ozone exposure for sporestrips without glassine envelopes;

[0058]FIG. 13 is a chart showing the variation of the fraction ofnegative biologic indicators with the duration of ozone exposure;

[0059]FIG. 14 is a flow diagram of a vessel for treating bulk grain withozone to kill fungi; and

[0060]FIG. 15 is a flow diagram of a multiport arrangement for treatinggrain with ozone as the grain passes through an auger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0061] Conventional water electrolysis devices utilize hydrogenformation (2H⁺+2e⁻->H₂) as the cathodic reaction. However, in an ozoneforming electrolysis device, there are considerable benefits toeliminating the hydrogen formation reaction and replacing it with oxygenreduction reactions at the cathode. The benefits of eliminating hydrogenformation in electrolysis systems for ozone generation include: (1)lower operating cell voltage; (2) elimination of hydrogen gasexplosions; (3) elimination of reactions between hydrogen and ozone inthe product; and (4) the ability to provide paired electrosynthesis ofozone at the anode and hydrogen peroxide at the cathode.

[0062] The electrochemical reactions of the present invention occur byapplying DC electricity between the anode and cathode. Water is fed tothe anode side where two competing water oxidation reactions take place;the thermodynamically favored oxygen (O₂) evolution reaction Equation(1) and the ozone (O₃) formation reaction Equation (2).

2H ₂ O→O ₂+4H ⁺+4e ⁻  EQUATION (1)

3H ₂ O→O ₃+6H ⁺+6e ⁻  EQUATION (2)

[0063] Utilization of high overpotentials, such as anode potentials muchgreater than 1.3 volts, and certain electrode materials enhance ozoneformation at the expense of oxygen evolution. The water oxidationreactions yield protons and electrons which are recombined at thecathode. Electrons are conducted to the cathode via the externalelectronic circuit. The protons are carried through a solid electrolyte,such as a proton exchange membrane (PEM), which is available from duPontunder the trademark “NAFION”.

[0064] The cathodic reactions involving the reduction of oxygen aregiven below:

O ₂+4H ⁺+4e ⁻→2H ₂ O  EQUATION (3)

O ₂+2H ⁺+2e ⁻ →H ₂ O ₂  EQUATION (4)

[0065] Specialized gas diffusion electrodes are required for thesereactions to occur efficiently. The presence of oxygen at the cathodesuppresses the hydrogen (H₂) formation reaction. Furthermore, the oxygenreactions are thermodynamically favored over hydrogen formation. In thismanner, the reduction of oxygen to either H₂O or H₂O₂ reduces theoverall cell voltage (i.e., the energy required to drive this system)below that required to evolve hydrogen (H₂) at the cathode in an aqueoussolution.

[0066] The proton exchange membrane placed between the anode and cathodeis made of a polymer material having sulfonate functional groupscontained on a fluorinated carbon backbone. Two such materials include a“NAFION” PEM having an equivalent weight of 1100 grams and a Dowexperimental PEM (XUS-13204.20) having an equivalent weight of 800grams. While “NAFION” 105, 115 and 117 will each operate satisfactorilyin the present invention, “NAFION” 117 is the preferred “NAFION”product. However, it is anticipated that a sulfonated polymer having anon-fluorinated carbon backbone would be operable according to thepresent invention. Such a polymer might include polystyrene sulfonate.Additionally, such a material might be coated with a fluorinatedmaterial to increase its resistance to chemical attack. It is alsoanticipated that a proton exchange membrane made of a polymer materialhaving carboxylate functional groups attached to a fluorinated carbonbackbone would be operable according to the present invention. Examplesinclude those available from Tokuyama Soda Company under the trademark“NEOSEPT-F”, Asahi Glass Company under the trademark “FLEMION”, AsahiChemical Industry Company under the trademark “ACIPLEX-S” and TosohCorporation under the trademark “TOSFLEX IE-SA48.” Further, polymericsystems based on: perfluoro bis-sulfonimides(CF₃—[CF₂SO₂NHSO₂CF₂]_(n)—CF₃); perfluoro phosphonic acids, and thecorresponding carbanion acids would function satisfactorily as protonexchange membranes according to the present invention. The Dowexperimental PEM gives much superior performance than the “NAFION” PEMmaterials, which are manufactured by DuPont. However, “NAFION” has beendetermined to be better for impregnating platinum electrodes.

[0067] The use of a PEM instead of a liquid electrolyte offers severaladvantages:

[0068] fluid management is simplified and the potential of leakage ofcorrosive liquids is eliminated;

[0069] the membrane serves as a separator between the anode and cathode;and,

[0070] the PEM/anode interface provides a chemical environment which issuited to the electrochemical ozone formation reaction.

[0071] The preferred PEMs contain perfluorinated sulphonic acids thatdisplay a very high resistance to chemical attack, such as “NAFION” 117and “NAFION” 115. Dow Chemical's experimental PEM XUS-13204.20 is themost preferred.

[0072] PEM-impregnated gas diffusion electrodes can be hot-pressed ontoboth sides of a purified proton exchange membrane, using a Carver hotpress, to produce a membrane and electrode (M&E) assembly. Thehot-pressing procedure involves placing a sandwich structure, consistingof the PEM and two electrodes—one at either side of the membrane—betweenthe platens of the press at approximately 100 psi, where the platenshaving been previously heated to 100° C. After the temperature of theplatens has been raised to within a preselected range of between 125° C.and 230° C., a preselected pressure in the range 1,000 psi to 50,000 psiis applied to the membrane and electrode assembly for a period of timevarying from 15 seconds to 150 seconds. The hot pressed M&E's should beimmediately removed from the hot press and mounted in an electrochemicalcell.

[0073] Preferred conditions for the preparation of M&E assemblies werefound to consist of a hot press temperature of 215° C., a hot pressingtime of 45 seconds and a hot press pressure in the range 3,000 psi to14,000 psi.

[0074] Lead dioxide anodes for use in the electrolytic cells of theinvention may be prepared by anodic deposition. The choice of anodicsubstrates on which lead dioxide is deposited are limited since mostmetals dissolve when deposition is attempted. However, the valve metals,such as titanium, titanium suboxides (such as that produced by AtraverdaLimited under the trademark “EBONEX”), platinum, tungsten, tantalum,niobium and hafnium are suitable as substrates for the anodes. Whentitanium, tungsten, niobium, hafnium or tantalum are utilized assubstrate materials, they are first platinized to eliminate passivationproblems sometimes encountered with the uncoated substrates.

[0075] Carbon in the form of graphite may be used as a substrate,however, lead dioxide adherence is a particular problem if the carbonhas not been thoroughly degassed. The carbon is degassed by boiling inwater for some time followed by vacuum drying over a period of days.When degassed, adherence is greatly improved with respect to thermalstress. Vitreous or glassy carbon does not appear to have the adherenceproblem.

[0076] Platinum is the most convenient substrate material to work with,gives the most uniform deposits, and does not present any additionalproblems. Platinum is therefore typically the most suitable substratematerial for lead dioxide anodes. However, its high cost may make otherpreviously mentioned substrate materials more practical for commercialuse.

[0077] In any event, lead dioxide is plated onto substrates in a wellknown plating bath comprising essentially lead nitrate, sodiumperchlorate, copper nitrate, and a small amount of sodium fluoride andwater. The substrate material is set up as the anode in a plating bathwith a pH maintained between 2 and 4. Current densities of between 16and 32 milliamperes per square centimeter give bright, smooth andadherent lead dioxide deposits. Bath temperature is most usuallymaintained at about 60° C. at all times during deposition. Thedeposition is carried out with vigorous stirring of the electrolyte andrapid mechanical vibration of the anode to give consistently finegranular deposits free from pinholes or nodules. A surface active agentmay be added to the plating solution to reduce the likelihood of gasbubbles sticking to the anode surface.

[0078] The limiting current for the cathodic reduction of dissolvedoxygen in either water or aqueous solution may be described as follows:$\begin{matrix}{i_{l} = \frac{\left( {n \times F \times D \times {Co}} \right)}{d}} & {{EQUATION}\quad (5)}\end{matrix}$

[0079] where:

[0080] i_(l) is the limiting current;

[0081] n is the number of electrons consumed per oxygen molecule reducedto water;

[0082] F is the Faraday constant (96,484 Coulombs);

[0083] D is the diffusion coefficient of oxygen in water (1.93×10⁻⁵cm²s⁻¹);

[0084] C₀ is the concentration of oxygen in water (1.41×10⁻⁶ molescm⁻³); and

[0085] d is the diffusion layer thickness. (For a static liquidelectrolyte, d is approximately 0.05 cm. For oxygen dissolved in arapidly flowing or well-stirred solution, d is about 0.005 cm.)

[0086] Using the values given above, i_(l) for the reduction ofdissolved oxygen in water is 2.1×10⁻³ Amps/cm². Given this low value ofi_(l), operation of an ozone electrolysis device with the reduction ofdissolved oxygen as the cathodic reaction is only possible at currentdensities considerably below those required for efficient ozonegeneration at the anode. A system which relies upon dissolved oxygen atthe cathode can produce high yields of ozone, but only if hydrogenformation occurs at the cathode. The full benefits of oxygen reductioncannot be incorporated into this type of system where liquid water isallowed to flow over the surface of the cathode electrode structure.

[0087] The electrochemical reactor of the present invention providespure air or oxygen gas to a gas diffusion cathode and feeds water onlyto the anode side.

[0088] The gas diffusion cathode consists of two layers: asemi-hydrophobic reaction layer (thickness of 5 μm to 100 μm) and ahydrophobic gas diffusion layer optionally having an imbedded metalliccurrent collector or a carbon cloth or carbon fiber paper. The preferredreaction layer for the reduction of oxygen at the cathode consists ofplatinum black with 30% PTFE. The gas diffusion layer consists of amixture of 4 parts carbon black and 6 parts PTFE with no platinumcatalyst and having an imbedded current collector, either a metalscreen, carbon cloth, or fibrous carbon paper. This layer acts as asponge to provide high concentrations of oxygen to the catalyst layer.In this manner, the rate of reaction at the cathode surface is no longerlimited by mass transport of oxygen through water. The rate of reactionis therefore increased and is limited only by the rate of reaction atthe catalyst surface/electrolyte interface. The use of the gas diffusionlayer, together with oxygen gas or air, avoids the production ofhydrogen gas and supports a high current density in the electrochemicalcell.

[0089] It is important to recognize that the PTFE-impregnated carbonpaper fiber is hydrophobic and therefore prevents saturation of the gasdiffusion layer with water. Additionally, solubilized PEM may be brushedonto the front surface of a catalyst layer. By applying the PEM inintimate contact with the three dimensional catalyst surface, the oxygenreduction reaction can take place in all three dimensions. This threedimensional bonding is accomplished by following the hot pressingtechnique described above. This technique, however, requirestemperatures near 220° C. which approaches the decomposition temperatureof the perfluorinated sulfonic acid material.

[0090] Typically, the PEM membrane separating the anode and cathode mustbe kept moist. This is necessary to hydrate the sulfonate sites on thepolymer to allow for proton transfer through the membrane. Usually themembrane is kept moist by humidifying the oxygen source to the cathode.However, one alternative means of hydrating the membrane includes theuse of tubes of approximately 4 to 9 mils diameter within the protonexchange membrane. These tubes are available from Perma-Pure, Inc. Inthis manner, water can be provided to the tubes along one edge of themembrane and spread throughout the membrane through capillary action.

[0091] An essential requirement of all ozone electrolysis processes isto operate at high current densities. This is because the efficiency ofozone generation is low at low current densities. At higher currentdensities, a higher proportion of the electrical current goes to thedesired ozone formation reaction at the expense of competing or sidereactions forming oxygen. The economics of ozone electrolysis systemsdictate the need for operation at high current density. Firstly, becausethe energy cost per unit amount of ozone generated is at a minimum andsecondly, the size of the electrodes, which determine the overallequipment cost, required to generate a given amount of ozone will be ata minimum when operating at high current densities where the efficiencyof ozone formation is the greatest.

[0092] The dependence of the electrochemical reactor voltage on currentdensity for various reactor temperatures is shown in FIG. 8. Cellvoltages were recorded 30 minutes after applying each current densityvalue to allow steady state condition to be reached. The reactortemperature was measured by a thermocouple probe located in the titaniumend plate. The cell voltage increased linearly with increasing currentdensity and decreased with increasing temperature at any selectedapplied current density. The variation of ozone current efficiency withreactor temperature for a number of current densities is given in FIG.9. Current efficiency is the proportion of the current supplied to thecell that goes toward the desired product ozone (O₃). Current efficiencywas determined by first finding the ideal yield if all the current goesto ozone production. The value of the ideal yield was then compared withthe actual ozone yield. The relationship given below is used todetermine the ideal yield: $\begin{matrix}{\frac{i \times t}{n \times F} = {{yield}\quad {in}\quad {moles}\quad {of}\quad {ozone}\quad {produced}\quad {assuming}\quad 100\% \quad {Coulombic}\quad {efficiency}}} & {{EQUATION}\quad (6)}\end{matrix}$

[0093] where:

[0094] i is the cell current in amps;

[0095] t is the time of electrolysis in seconds;

[0096] n is the number of electrons taking part in the reaction(6); and

[0097] F is the Faraday constant (96,484 Coulombs).

[0098] The current efficiency versus temperature profiles go through amaximum at approximately room temperature for all current densities. Theprofiles presented in FIG. 9 show that the current efficiency maximizesat 18 to 19 percent on operating the electrochemical reactor at acurrent density of 1.6 to 2.0 amps per square centimeter and atemperature of 23° C.

[0099] The DC electrical energy requirement (J) for ozone production inkilowatt hours per kilogram (kWh/kg) of ozone is given by the followingequation (7): $\begin{matrix}{J = \frac{E \times n \times F}{3600 \times N \times m}} & {{EQUATION}\quad (7)}\end{matrix}$

[0100] where:

[0101] E is the cell voltage;

[0102] n is the number of electrons released per mole of ozone formed(6);

[0103] N is the current efficiency; and

[0104] M is the molecular weight of ozone (48 grams).

[0105] As can be seen from the above figures, the reaction rate, ozonecurrent efficiency and conductivity of the PEM electrolyte are dependentupon temperature. Temperature is typically controlled by the circulationand heat exchange of the water flowing to the anode.

[0106] One advantage of the electrochemical methods of the presentinvention is the ability to provide paired synthesis or pairedelectrosynthesis. The term “paired synthesis” means that the productgenerated at both the anode and cathode may be used together in a commonapplication. For example, the present invention allows for selectivegeneration of hydrogen peroxide, H₂O₂, at the cathode and generation ofozone, O₃, at the anode. Both of these desired products may be collectedtogether and used in various applications, such as the treatment ofwaste water streams. Therefore, the process can generate twice as muchoxidant product as was possible before.

[0107] According to the present invention, an electrochemical cell forthe production of ozone may be constructed by placing a proton exchangemembrane between an anode and a cathode. The cathode may be coated witha semi-hydrophobic catalyst material supported on a layer of carbonpaper fiber impregnated with a sintered mass derived from fine carbonpowder and Teflon emulsion. The anode is exposed to a source of waterand the gas diffusion cathode is exposed to oxygen gas or air. Anexternal electrical circuit is necessary to provide electron flow fromthe anode to the cathode, while the PEM provides for proton flow. Also,the electrochemical cell must provide a means for collecting theproduced ozone.

[0108] The present invention also encompasses the use of a multiple cellarrangement, otherwise referred to as “filter press arrangement.” Thisarrangement stacks individual cells back to back, in series, so that thecurrent or electricity flows through one cell to the next. Each cell isseparated by a bipolar plate which allows the electricity from the anodeof a first cell to pass through to the cathode of a second cell. Eachcell must also be provided with a source of water to the anode surfaceand a source of oxygen gas to the gas diffusion cathode. Theserequirements of the present invention may be accomplished throughnumerous embodiments. The preferred embodiment for carrying out themethod of the present invention is disclosed as follows:

[0109] Referring to FIG. 1, a cross-sectional side view of a depolarizedozone electrolysis cell 10 is shown. A proton exchange membrane (PEM) ora solid polymer electrolyte (SPE) 12, such as a perfluorinated sulfonicacid polymer, is disposed in the center of the cell. Bonded to one side(the cathodic side) of the solid electrolyte 12 is the electronicallyconducting, semi-hydrophobic, oxygen reduction electrocatalyst layer 14of the gas diffusion cathode. This electrocatalyst layer 14 may becomprised of Teflon-bonded platinum black or carbon-supported highsurface area platinum. The gas diffusion layer 16 of the gas diffusionelectrode is integrally formed onto the catalyst layer 14.

[0110] On the other side (the anodic side) of the solid electrolyte 12is an anode made up of a catalyst layer 18 formed on a substrate 20. Theelectronically conducting, hydrophilic, ozone forming electrocatalystlayer 18 is made of lead dioxide (PbO₂) or a platinum-tungsten alloy(Pt/W). The substrate 20 is a porous, non-corroding, electronicallyconducting support material that is preferred to be fabricated usingsintered titanium (or tantalum) particles.

[0111] Two non-conducting gaskets 17 and 21 are placed on either side ofthe solid electrolyte 12. The gaskets 17 and 21 have cutouts to fitaround the perimeter of the cathodic gas diffusion electrode and theanodic substrate 20, respectively. The gasket should have a thicknessgreater than that of the cathodic gas diffusion electrode and the anodicsubstrate 20 so that it may be sufficiently compressed to seal liquidsand/or gases.

[0112] The cathodic chamber of electrochemical cells of the presentinvention is surrounded and sealed with gasket materials well known toone skilled in the art. Gasket materials can be selected from the groupconsisting of neoprene, silicone rubber elastomer materials, Viton,Kalrez, and urethanes. The elastic nature of these materials compensatefor any contraction/expansion encountered in electrochemical cells ofthe present invention under various operating conditions. Because of thehighly oxidizing aggressive environment encountered in the anodic sideof electrochemical cells, gaskets will be selected from the group offluorocarbon-based polymeric materials consisting ofpolytetrafluoroethylene (PTFE or Teflon), chlorotrifluoroethylene,polytetrafluoroethylene containing organic or organic fillers, copolymerof tetrafluoroethylene and hexafluoropropylene (FEP), polyvinylidenefluoride (PVDF), and fluorocopolymers containing vinylidene fluoride andhexafluoropropene.

[0113] A cathodic depolarizer, typically a gas such as oxygen, entersthe cell through port 22 which is in fluid communication with a seriesof channels 24 for fluid flow to the gas diffusion layer 16 of thecathode. The depolarizer gas and the products generated, such as liquidhydrogen peroxide, flow out of the cell 10 through port 26. Thedepolarizer flow is typically downward across the cathode so that theliquids generated can more easily be removed from the cell.

[0114] In a similar manner, water is fed into the cell 10 for exposureto the anode electrocatalyst layer 18 through port 28. The water flowsthrough the channels 30 and out the port 32. The water flow is typicallydirected upward across the anode so that gaseous oxidation products donot become trapped in the cell.

[0115] The structure of the cell 10 is held together with two metal endplates 34 and 36. The metal end plates can be selected from the group ofmetals consisting of iron, nickel, copper, aluminum, stainless steels,Monel, Inconel, Hastelloy, titanium, tantalum, hafnium, niobium andzirconium. The preferred metal is titanium. The surfaces of the endplates can be plated with a noble metal selected from the groupconsisting of platinum, gold, palladium, ruthenium and iridium. The endplates 34 and 36 are secured together by a plurality of cell tie rods 38having male threads and a plurality of nuts 40. To keep the endplates 34and 36 electrically isolated from each other, a plurality ofelectrically insulating sheaths 42 and washers 44 are used inconjunction with each rod 38 and nut 40, respectively. Havingelectrically isolated the endplates 34 and 36, a positive terminal orbusbar 46 and a negative terminal or busbar 48 can be connected to a DCpower source (not shown).

[0116] Referring now to FIG. 2, a second embodiment of the presentinvention is shown. The electrochemical cell 50 is substantially similarto the cell 10 of FIG. 1, except that the channels 24 and 30 (shown inFIG. 1) have been substituted with non-corroding, electronicallyconducting, expanded metal 52 and 54, respectively. The expanded metalincludes either an expanded metal sheet, a metal wire mesh or a metalfoam. The expanded metal 52 and 54 still allow flow from port 22 to port26 and from port 28 to port 32, respectively, but provides a moreturbulent flow pattern and leave a greater portion of the electrodesurface area exposed to fluid flow. Also note that the gaskets 56 and 58have a compressed width equivalent to the anodic substrate 20 plus theexpanded metal 54 or the cathodic gas diffusion electrode plus theexpanded metal 52. A slot 59 has been cut in the gasket to allow flowfrom between the ports.

[0117] Now referring to FIG. 3, the variation of electrochemical cellvoltage with current density applied to the anodic and cathodicelectrodes of the cell represented schematically in FIG. 2 illustrated.It can be seen from FIG. 3 that the cell voltage, hence, the electricalenergy consumed, decreases with increasing cell temperature for anygiven applied current density. In obtaining the data represented by FIG.3, pure oxygen gas, as a cathodic depolarizer, was fed into the cathodechamber of the cell represented schematically in FIG. 2 under a pressureof 40 psi. At the same time, water was recirculated over the anodesurface under atmospheric pressure. The low cell voltages obtained onusing oxygen gas as a cathodic depolarizer give rise to a considerableimprovement over the corresponding cell voltages described in the stateof the art.

[0118] Now referring to FIG. 4, the dependence of ozone formationcurrent efficiency on the current density applied to the anode andcathode of the cell represented schematically in FIG. 2 is illustrated.High current efficiencies are only achieved as a result of applying highcurrent densities which normally give rise to high cell voltages.However, with the present invention, the use of a cathodic depolarizer,for example, oxygen gas in the case, enables high current densities tobe reached while maintaining low cell voltages. A high ozone currentefficiency is desirable so as to make the apparatus as compact and asefficient as possible.

[0119] Now referring to FIG. 5, a cross-sectional view of a multiplecell arrangement 60 of the depolarized ozone electrolysis cell of FIG. 2is shown, where the cells are electrically connected in a seriescircuit. The same numbering system as used in FIG. 1 and FIG. 2 has beenincorporated into FIG. 5. Accordingly, FIG. 5 shows sevenelectrochemical cells separated by six bipolar plates 62. The bipolarplate 62 of the present invention can be made from a metal selected fromthe group of valve metals consisting of titanium, tantalum, hafnium,zirconium or from the group of metals known as stainless steels whichinclude SS304, SS316 and other high chromium/nickel-cantaining alloys.The bipolar plate can have a thickness in the range of 0.2-2 mm (8-80mils). Both flat surfaces of the bipolar plate may be plated with a thinlayer of a noble metal selected from the group containing platinum, goldpalladium and iridium. The bipolar plate has sections removed forinternal manifolding to allow fluid flow between adjacent cells.

[0120] The inlet port 28 and outlet port 32 are now in communicationwith inlet manifold 64 and outlet manifold 66, respectively, which arecomprised a series of slots cut into the gaskets. In this manner, wateris delivered to the anode 20 of each individual cell of the multiplecell arrangement 60 via manifold 64, carried over expanded metal 54 ofeach individual cell, collected via manifold 66 and removed through port32. A similar manifold system for the cathode is positioned at 90degrees of rotation from that for the anode.

[0121]FIG. 6 shows an apparatus 70 for carrying out a process for theelectrochemical production of ozone gas and/or ozonated water. Anelectrochemical cell 72 is used for the production of ozone from wateraccording to the apparatus of FIG. 5. Water is input to the cell 72through port 28 and is removed through port 32, as described previously.The water exiting port 32 contains entrained ozone gas. The ozone-ladenwater flows into a gas/liquid separator tank 74 via pipe 76. The ozonethen disengages from the water 80 and rises into the vapor space 82. Theozone is removed from tank 74 through outlet pipe 84. Makeup deionizedwater may be added to the tank 74 through pipe 86.

[0122] The water 80 is withdrawn from tank 74 through pipe 88. The waterin pipe 88 may be either discarded by directing flow through valve 90 topipe 92 or recycled by directing flow to the recycle line 94. Therecycle line 94 passes through a pump 96 before feeding the water intoport 28.

[0123]FIG. 7 is a schematic diagram of the apparatus of FIG. 6 incombination with ancillary equipment used in a process for thepurification of wastewater 104. The apparatus 100 is identical to thatof FIG. 6 with the addition of a vessel 102 for purifying water. Ozoneproduced in electrochemical cell 72 and separated in tank 74 isdelivered to vessel 102 through pipe 84. The ozone is evenly distributedin the vessel 102 through a porous, non-corrosive metal tube 106,preferably made of titanium, for introducing very fine O₃/O₂ gas bubblesinto the water 104. Ozone bubbles rise up through the water 104 andcontact certain organic and biological material. The ozone, which maycontain hydrogen peroxide, is combined with ultraviolet light fromserpentine-shaped UV lamp 108 to produce hydroxyl radicals which reactnon-selectively with dissolved organics to cause their completeoxidation to carbon dioxide (CO₂). Oxygen and carbon dioxide are thenreleased from the vessel 102 through exhaust pipe 110. The wastewatermay be purified either as a batch process or continuously as will beapparent to one with ordinary skill in the art.

[0124] It will be apparent to one with ordinary skill in the art thatthe methods of the present invention may be accomplished throughnumerous equivalent embodiments including various geometricconfigurations.

[0125] The method of the present invention can include the disengagementof the product ozone gas from water. However, this can be easilyaccomplished in a vessel of short residence time where the ozone willquickly disengage from the water. In this manner, ozone may be collectedin a concentrated gaseous product which facilitates its use in numerousapplications. For example, the product ozone of the present inventionmay be applied to severe waste water applications. Prior art devicessuch as those taught by Stucki require the intimate contact of the wastewater with the anode. This caused numerous problems as the contaminantsof the waste water would foul the anode and reduce cell efficiency. Thisproblem has been overcome by the present invention.

EXAMPLE #1

[0126] Referring to FIG. 2 and to FIG. 6, a trial run of the disclosedinvention was practiced. Lead dioxide, electroplated onto a sinteredporous titanium substrate (Astro Met, Inc.), was used as the anodicelectrode material. Prior to applying the lead dioxide electrocatalystlayer, the porous titanium substrate was first cleaned by glass beadblasting, followed by sonication in water, and plated with a thin layerof platinum, using a commercial plating solution (EngelhardCorporation). A commercially available platinum-catalyzed gas-diffusionelectrode (ELAT, E-TEK, Inc.) was used as the cathodic electrodematerial. The anodic and cathodic structures each had an active area of25 cm². The anodic and cathodic electrocatalyst layers were impregnatedwith a 5 wt % Nafion® solution in a mixture of lower aliphatic alcoholsand 10% water (obtained from Aldrich Chemical Company) and dried,yielding PEM loadings of ˜0.6 mg. cm⁻². The PEM-impregnated cathodicgas-diffusion electrode was bonded to one side of a precleaned Dowexperimental proton exchange membrane (XUS-13204.20) by means ofhot-pressing under optimum conditions. The PEM-impregnated leaddioxide-plated porous titanium substrate (as the anodic electrode) wasplaced on the other side of the Dow proton exchange membrane.

[0127] The membrane and electrode assembly obtained, along with twopieces of platinum-plated expanded titanium metal (where one each of theexpanded metals was in contact with the other planar surfaces of theanodic and cathodic electrodes), together with two Teflon gaskets, wereinserted between two platinum-plated titanium endplates of a singleelectrochemical cell, representative of that shown schematically in FIG.2. The chemically stable Teflon gaskets were used to seal the cellcomponents on bolting the endplates, together with electricallyinsulated bolts and nuts, using a torque of 50 inch-pounds. Electricalconnections between the positive and negative endplates and aHewlett-Packard model 6572A DC power supply were made, using insulatedwire leads. Two additional screws, placed on the sidewall of eachendplate, allowed the measurement of cell voltages, using a Fluke (model8050A) digital multimeter.

[0128] Performance characterizations of the single electrochemical cellwere carried out using the test apparatus shown in FIG. 6. Teflonswagelok fittings and Teflon tubing were used in connecting theelectrochemical cell to the source of pressurized oxygen gas,Teflon-lined pump (Cole-Parmer micropump model 020-000), water/gasseparator (water reservoir) and gas-phase ozone analyzer. The water/gasseparator (water reservoir) was made of Pyrex glass. Pure oxygen gas asa cathodic depolarizer was supplied in the manner of a single pass tothe cathodic inlet port from a pressurized cylinder at a pressure of 40psi. To maintain a pressure of 40 psi within the cathode space in theelectrochemical cell, a back-pressure regulator set at 40 psi wasconnected to the cathodic outlet port. While maintaining a constantpressure of 40 psi, an oxygen gas flow rate through the cathode space ofthe electrochemical cell of 500 ml min⁻¹ was achieved. Water wasrecirculated continuously over the surface of the anode at a flow rateof 200 ml min⁻¹ from the water reservoir which contained 300 ml ofwater. The return water recirculation loop passed through a heatexchanger (Astro Metallurgical, Inc.; ACX heat exchanger, model 4X8-14)before entering the anodic inlet port.

[0129] An ozone monitor (Ozone Research and Equipment Corporation, model03M-110) was used for measuring gas-phase ozone concentrations. Theozone-containing gas stream was separated from the fluid flow exitingfrom the anodic outlet port in the water/gas separator at atmosphericpressure. The gas stream was then fed into the inlet of the ozonemonitor under atmospheric pressure. This instrument measures theabsorption of UV light by ozone at 254 nm and provides a directdetermination of ozone concentrations in terms of mg/standard liter ormg min⁻¹ Knowing the amount of electrical charge passed during theelectrochemical formation of ozone allowed a determination of itscurrent efficiency to be made.

[0130] The experimental parameters varied systematically were theapplied current density and electrochemical cell temperature. Thecorresponding parameters measured were the cell voltage and theconcentration of ozone in the gas phase. The dependence ofelectrochemical cell voltage on current density for various celltemperatures is shown in FIG. 3. Cell voltages were recorded 60 minutesafter applying each current density value, so as to allow steady stateconditions to be reached. Constant electrochemical cell temperatureswere maintained by circulating water from a large constant temperaturewater bath (Lauda model K-4IRD) through the heat exchanger. The celltemperature close to the PEM/electrode interfaces was measured by meansof a thermocouple probe placed in a thermowell that was drilled into thesidewall of one of the titanium endplates at a location close to theelectrode surface. It can be seen from FIG. 3 that the cell voltageincreases rapidly at low current densities followed by a more gradualincrease at higher current densities. However, the cell voltagedecreases with increasing temperature at any given applied currentdensity. The cell voltage is a combination of the reversible cellvoltage, overpotential losses at each electrode and ohmic dropsinternally within the M&E assembly and in the external electricalconnections.

[0131] The variation of ozone current efficiency with applied currentdensity for a number of electrochemical cell temperatures is presentedin FIG. 4. For all current densities, the ozone current efficiencieswere highest for the lowest cell temperature and decreased for any givencurrent density with increasing temperature. At the lowest temperature,the ozone current efficiency-current density profile approached amaximum current efficiency of the order of 15% at a current density ofapproximately 2.5 A cm⁻².

[0132] It is apparent from the profiles given in FIG. 3 that the energyrequired to impress a given current density between the anode/protonexchange membrane/cathode sandwich decreases with increasingelectrochemical cell temperature. The profiles given in FIG. 4 showthat, at high current densities, the ozone current efficiency is onlyslightly affected with increasing electrochemical cell temperature.

[0133] At a current density of 1.6 A cm⁻², the rate of ozone productionby the electrochemical cell was 25 mg of O₃ per minute. Taking theelectrochemically active dimensions of the cell (5 cm×5 cm×0.6 cm),yields a volume of 15 cm³. This illustrates the compact nature of theelectrochemical cell of the present invention. Since single cells of thepresent invention can be stacked in series to form a unitary structure,as represented schematically in FIG. 5, further stacking will result ina high ozone output within extraordinarily compact dimensions.

EXAMPLE #2

[0134] Referring now to FIG. 1 and FIG. 6, another example of theperformance derived from the disclosed invention is outlined. The anodicelectrode was prepared in a manner identical to that described inExample #1 above. However, a PEM-impregnated high surface area palladiumblack-catalyzed, gas-diffusion electrode was used as the cathodicelectrode material. The palladium electrocatalyst layer was prepared byadding 3 ml of water to 1.68 g of high surface area palladium blackpowder (Johnson Matthey, Inc.) and sonicated for 20 minutes. 0.22 g ofTeflon emulsion (available from duPont) was added to this mixture,followed by 20 minutes of additional sonication, the mixture was thenapplied to one side of a commercially available gas-diffusion electrode(E-TEK, Inc.) and heated in an inert atmosphere at 350° C. for threehours.

[0135] The PEM-impregnated anodic and cathodic electrode structures,each having an active area of 5 CM², were bonded on either side of aprecleaned segment of Nafion 117 PEM material. The membrane andelectrode assembly obtained, together with two Teflon gaskets, wereinserted between titanium endplates of a single electrochemical cell,represented schematically in FIG. 1. The inner surfaces of the titaniumendplates were electroplated with a thin platinum film to prevent poorlyelectronically conducting oxide film growth on the surfaces of theendplates. Humidified air was supplied to the inlet port of the cathodicendplate from a pressurized air cylinder. A back pressure regulator wasconnected to the cathodic outlet port and set to give an air pressurewithin the cathode space of the electrochemical cell of 80 psi, whileallowing an air flow rate of 500 ml min⁻¹. Water was recirculatedcontinuously over the back surface of the anode at a flow rate of 150 mlmin⁻¹. The anodic water reservoir had a volume of 500 ml.

[0136] Performance characterizations of the single electrochemical cellwere carried out using the experimental test apparatus shown in FIG. 6and as described in Example #1 above, except that electrical connectionson the outside surfaces of the titanium endplates were made to aHewlett-Packard (model 6282A) DC power supply. Cell voltages wererecorded 30 minutes after applying each current density value, so as toallow steady state conditions to be reached. The dependence ofelectrochemical cell voltage on current density for various celltemperatures is shown in FIG. 8. The cell voltages increased almostlinearly with increasing current density and decreased with increasingtemperature for any given applied current density.

[0137] Dissolved hydrogen peroxide in water electrosmoticallytransported through the proton exchange membrane was sampled at the exitport from the cathode endplate, shown in the schematic given in FIG. 1.Hydrogen peroxide concentrations were measured using an Orbeco AquaAnalyzer II spectrophotometer. Dissolved ozone concentrations weredetermined by colorimetry at 600 nm, using samples of ozonated waterwithdrawn from the water/gas separator, shown in the schematic given inFIG. 6. The highest dissolved ozone concentrations in water, measured atatmospheric pressure, were in the range 10-20 ppm. The values ofdissolved ozone were influenced considerably by the temperature of theelectrochemical cell. Ozone gas concentrations were determined, asdescribed in Example #1 above, using gas samples withdrawn from thewater/gas separator, as illustrated in FIG. 6.

[0138] The variation of ozone current efficiency with electrochemicalcell temperature for a number of current densities is given in FIG. 9.the current efficiency-temperature profiles go through a maximum atapproximately room temperature for all current densities. This is aconsiderable advantage for the PEM-based electrochemical cell of thepresent invention. The profiles presented in FIG. 9 show that thecurrent efficiency maximizes at 18-19% on operating the electrochemicalcell at a current density of 1.6-2.0 A cm⁻² and a temperature of 25° C.

[0139] The electrochemical cell, represented schematically in FIG. 1,yielded an ozone production rate of 5 mg of O₃ per minute. The cell hadelectrochemically active dimensions of 2.24 cm×2.24 cm×0.3 cm giving avolume of 1.5 cm³. Again; this example illustrates the compact nature ofthe electrochemical cell of the present invention for the production ofozone gas.

[0140] Variation of the hydrogen peroxide production rate with appliedcurrent density on operating the electrochemical cell representedschematically in FIG. 1 in a paired oxidant synthesis manner is given inFIG. 10. The hydrogen peroxide production rate increases with increasingcurrent density and approaches a maximum value at an applied currentdensity of 1.6 A cm⁻². Because of the low hydrogen peroxide productionrates, calculated current efficiencies were very low and, in all cases,were less than 1%.

[0141] Another embodiment of the present invention may be referred to asan electrochemical cell for the production of ozone incorporating acathodic depolarizer. A cathodic depolarizer totally eliminates hydrogenevolution at the cathode and may lower the cell voltage required toproduce ozone.

[0142] Application of a DC source of electrical energy to twoelectronically conducting electrodes immersed in an aqueous electrolytecan bring about the decomposition of water molecules into theirconstituent elements, namely hydrogen and oxygen gases. This process isparticularly favored if the anions and cations associated with theelectrolyte do not undergo electrochemical reactions at theelectrode/solution interfaces. Aqueous solutions of acids, salts andbases are most commonly used as electrolyte solutions. The mineralacids: sulfuric; phosphoric; tetrafluoroboric and hexafluorophosphoricare particularly suitable. It is also anticipated that phosphonic,sulfonic, perfluoro bis-sulfonimides and the corresponding carbanionacids in monomeric, dimeric or oligomeric forms would be operableaccording to the present invention. Supplying an external source of DCelectrical energy to an electrochemical cell that brings about thedecomposition of water is referred to as an electrolysis process and theelectrochemical cell is referred to as an electrolysis cell. In order tominimize heating effects within the electrolysis cell and, hence, tolower the consumption of electrical energy, the positive and negativeelectrodes are placed as close as possible to each other without shortcircuiting taking place. In order to minimize the space between thepositive and negative electrodes, a separator material is usually placedbetween them. Separators are thin film materials, either inorganic(asbestos) or organic (Daramic or Celgard) in nature, and are electricalinsulators containing microporous channels or pathways that allow flowof ions through the material. A requirement for a separator to be usedis that it should be well wetted by the electrolyte solution in theelectrolysis cell and should be chemically and electrochemically stable.

[0143] In aqueous acid solutions, the decomposition of water involvestwo electrochemical reactions which take place at the positive andnegative electrodes. At the positive electrode, water molecules areoxidized, liberating oxygen gas and protons (which transport ioniccurrent in the solution) and electrons which flow through the externalcircuit and power source to the negative electrode. This electrochemicalreaction is represented by Equation [8] and has a standard electrodepotential of 1.23 V.

2H ₂ O→O ₂+4H ⁺+4e ⁻ ;E°=1.23V(25° C.)  EQUATION (8)

[0144] At the negative electrode, the protons recombine with theelectrons to liberate hydrogen gas, which is represented by Equation [9]and has a standard reversible potential of 0.00 V.

4H ⁺+4e ⁻→2H ₂ ;E°=0.00V(25° C.)  EQUATION (9)

[0145] If the nature of the catalytic surface of the positive electrodeis changed, a competing electrochemical water oxidation reaction maybecome more favorable. This competing water oxidation reaction involvesthe liberation of ozone gas and is represented by Equation [10] whichhas a reversible potential of 1.51 V.

3H ₂ O→O ₃+6H ⁺+6e ⁻ ;E°=1.51V(25° C.)  EQUATION (10)

[0146] Based on thermodynamic criteria, it is apparent from Equations[9] and [10] that the minimum cell voltage required to decompose waterelectrochemically into hydrogen and ozone gases under standardconditions requires a minimum of 1.51 V to be applied between thepositive and negative electrodes. Due to electrical resistance in theelectrolyte solution between the positive and negative electrodes andthe overpotentials required in order to make reaction [9] and [10]proceed at significant rates at 25° C., the actual cell voltage will beon the order of 3.0 V.

[0147] As a first step to lower this actual cell voltage and, hence,minimize the consumption of electrical energy, the electrodes need to beplaced as close as possible to each other. This is greatly facilitatedby the use of an ion exchange membrane, such as the proton exchangemembrane, which can have thicknesses in the range 50-175 μm (2-7 mils).Use of an ion exchange membrane can yield a so-called “zero gap” betweenthe positive and negative electrodes. It is also advantageous if the ionexchange membrane is a proton-conducting membrane, such perfluorinatedsulfonic acid polymer sold by DuPont under the trademark “NAFION” 117,which would enable the electrochemical reaction to proceed as describedby Equations [9] and [10].

[0148] A second step in lowering the cell voltage and, hence, minimizingelectrical energy consumption, is to coat the positive and negativeelectrodes with electrocatalyst layers that speed up the rates of thehydrogen evolution reaction and the ozone formation reaction. It is wellknown that platinum is the most effective electrocatalyst for hydrogenevolution, particularly in acid solutions, and that lead dioxide ishighly effective for the electrochemical formation of ozone from water.However, placing the electrodes as close as possible to each other andusing the most effective anodic and cathodic electrocatalysts may lowerthe cell voltage only a few hundred millivolts.

[0149] It is almost impossible using these approaches to reduce the cellvoltage for the electrochemical production of ozone from water muchbelow 3.0 V. A different result is achieved if the cathodic reductionproduct is not hydrogen but involves another reaction leading to adifferent product. Suppose that the hydrogen evolution reaction isreplaced by a cathodic reduction reaction taking place at a morepositive potential than that at which hydrogen is evolved; then thereversible potential for this reaction would be more favorable and thecell voltage would be reduced. The selection criterion for chemicalsthat could partake in an alternative cathodic reduction reaction, thatis, function as cathodic depolarizers, is that they have a thermodynamicreversible potential more positive than that corresponding to theevolution of hydrogen gas, which is represented by Equation [9]. It isalso desirable that these cathodic depolarizers be abundantly available,or readily reoxidizeable, for recycling into the electrolysis cell.

[0150] Examples of potential cathodic depolarizers that could be used inan electrolysis cell containing an acidic electrolyte and which could becombined with the anodic oxidation of water, liberating ozone gas, aregiven in Table 1, including oxygen. As seen from the table, some ofthese cathodic depolarizers exist in the gas phase, while others areacids or salts and can be exploited only when dissolved in water. All ofthe cathodic depolarizers identified in Table 1 support highelectrochemical reaction rates, that is high current densities at lowoverpotentials. This is particularly true if these depolarizers areutilized in conjunction with-appropriate cathode electrode structures.In the case of the gas-phase cathodic depolarizers, a gas-diffusionelectrode would be most suitable. However, for the cathodic depolarizersthat can only be used when dissolved in water, flow-by, flow-through,packed-bed and fluidized-bed electrode structures are more advantageousin order to achieve high current densities.

[0151] The gaseous-phase cathodic depolarizers could be supplied to thenegative electrode under pressure, which would further increase theelectrochemical reaction rate, that is, allow even higher currentdensities to be realized. The use of a proton exchange membrane as theTABLE 1 Theoretical Rever- Cell sible Voltage Poten- with O₃ De- tialEvolution Cathodic polarizer Cathodic at as Anodic Depolarizer PhaseReaction 25° C. Reaction Chlorine Gas Phase Cl₂ + 2c⁻ → 2 Cl⁻ 1.36 V0.15 V Bromine Gas Phase Br₂ + 2c⁻ → 2Br⁻ 1.09 V 0.42 V Chlorine GasPhase ClO₂ + H⁺ + c⁻ → 1.277 V 0.23 V Dioxide HClO₂ Dinitrogen Gas PhaseN₂O₄ + 4H⁺ + 4c⁻ → 1.035 V 0.475 V Tetroxide 2NO + 2H₂O Oxygen Gas PhaseO₂ + 4H⁺ + 4c⁻ → 1.23 V 0.28 V 2H₂O Air Gas Phase O₂ + 4H⁺ + 4c⁻ → 1.23V 0.28 V 2H₂O Ferric Aqueous Fe³⁺ + c⁻ → Fe²⁺ 0.77 V 0.74 V ChlorideSolution Benzo- Aqueous BQ + 2H⁺ + 2c⁻ → 0.70 V 0.81 V quinone SolutionHQ Hypo- Aqueous HBrO + H⁺ + 2c⁻ → 1.33 V 0.18 V bromous Solution Br +H₂O Acid Hypo- Aqueous HClO + H⁺ + 2c⁻ → 1.48 V 0.03 V chlorous SolutionCl⁻ + H₂O Acid Sodium Aqueous [Fe(Cn)₆]³⁻ + c⁻ → 0.36 V 1.15 V Ferri-Solution [Fe(CN)₆]⁴⁻ cyanide Sodium Aqueous NO₃ + 3H⁺ + 2c⁻ → 0.93 V0.58 V Nitrate Solution HNO₂ + H₂O

[0152] electrolyte greatly facilitates the use of pressure onintroducing the cathodic depolarizer into the electrochemical cell,since it will prevent the removal of electrolyte from the cathodechamber, as would occur with a liquid electrolyte solution, and willallow the electrolysis cell to function, even with different pressuresin the cathode chamber and in the anode chamber. Furthermore, the ozoneproduct can be delivered out of the cell under pressure suitable forimmediate use in pressurized applications. Since the proton exchangemembrane also functions as a separator, it prevents transfer of thecathodic depolarizer and its electrochemical reduction products fromdiffusing or migrating into the anode chamber, where they couldinterfere with the evolution of ozone gas from water.

[0153] The ozone produced by the electrochemical synthesis of thepresent invention may be utilized either in aqueous solutions directlyfrom the cell or after disengaging the ozone gas from the water. Aqueousozone may be preferred in such applications as wastewater treatment andpurification. The aqueous cell effluent containing solubilized ozone gasis added into a wastewater stream containing organic substances wherethe ozone gas can react with the organic substances. The reactionmechanism, as described above, may optionally be assisted by exposingthe ozone-containing wastewater stream to ultraviolet radiation whichpromotes the formation of hydroxyl and peroxyl radicals. It is preferredthat a residual of ozone be maintained in the wastewater stream untilimmediately prior to use, at which time the ozone should be eliminatedfrom the water. The ozone can be eliminated either by decomposition ordisengagement.

[0154] Some ozone applications require that the ozone gas be separatedfrom the water and applied in the gas phase. Typically, the ozone gaswill be fed to a chamber where the ozone gas can react with a givenreactant material. In applications requiring high concentrations ofozone, this chamber may be evacuated of air to avoid dilution of theozone. Following completion of the reaction, the ozone is eliminatedfrom the chamber and the chamber is filled with air to allow safehandling and removal of sterilized items. Ozone may be eliminated fromthe chamber by decomposition or by evacuation of the ozone gas.

[0155] In order to improve the conditions for sterilization ordisinfection of biological materials and residues within the chamber,the temperature, pressure and humidity inside the chamber may becontrolled. Control of conditions in the chamber is most important wherethe chamber is a chemical reaction vessel. In a chemical reactionvessel, the ozone will react with other chemical reactants to produce acommercial product rather than the sterilization of biologicalmaterials.

Sterilization and Disinfection

[0156] A second application for the ozone of the present invention isthe sterilization of medical/dental tools and apparatus. Currently,medical instruments are sterilized in ethylene oxide-containing cabinetswhere the instruments are exposed to ethylene oxide for a period of 4 to5 hours. The disadvantages of such systems include the fact that theethylene oxide is extremely flammable in any concentration when combinedwith oxygen and is also a carcinogen. In addition, under certainconditions, such as a battlefield, there is insufficient time to allowadequate sterilization of instruments. Therefore, there is a need forquick and efficient sterilization of medical instruments as would beprovided by the highly concentrated ozone of the present invention.

[0157] An ozone sterilization chamber was built by completely refittinga surplus AMSCO EtO sterilizer (Model AM-23, American SterilizerCompany, Erie, Pa.), with polytetrafluoroethylene gas lines, electricalwiring, new cycle timers, polytetrafluoroethylene gaskets, and a vacuumair purge system. The only portions of the original device that weresalvaged were the stainless steel chamber and the exterior heatingblanket (this required a new controller). A 400 cm² active electrodearea ozone generator with a gas feed rate of 2.2 liters per minute (1pm) was used for a breadboard sterilizer. The ozone concentration in theoutput from the generator was between 10 and 12 wt %. The ozonegenerator had a power consumption of 2.4 kilowatts. Total volume of thesterilizer chamber was 40 liters. Elevated humidity in the chamber wasachieved by keeping an open container of distilled H₂O inside thechamber while cycling the sterilizer, further the ozone/oxygen mixturegenerated by the electrochemical reactor is in a near-water saturatedstate when it leaves the ozone generator. The air exhaust systemutilized a piston air pump. The ozone concentration was measuredspectrophotometrically at 254 nm using a Shimadzu UV spectrophotometer(Model UV-2101PC, Shimadzu Corp., Japan), integrated with a computer fordata evaluation.

[0158] The sterilizer is normally operated with a continuous feed ofozone gas into (and out of) the chamber. This is referred to as the“flow through” mode. There are three phases of the sterilizationcycle: 1) evacuate; 2) sterilize; and, 3) flush. Each phase of the cycleis described below.

[0159] After samples have been loaded and the chamber door is secured,the evacuation is initiated through a vacuum block valve which opens toa vacuum pump. The chamber air is evacuated through the vacuum pump andout an air purge valve. After approximately 5 minutes, a vacuum of 30″Hg is reached.

[0160] Sterilization can be initiated after about 5 minutes ofevacuation. The vacuum pump is turned off and the vacuum block valve isclosed prior to introducing ozone into the chamber. Ozone gas is made toflow continuously through the chamber at approximately 2 liters perminute. The pressure in the chamber may be maintained close toatmospheric, but it is preferred that the pressure be greater than about20 psi. The length of the sterilizing phase is also the duration ofozone exposure of the samples in the chamber. Testing of the sterilizerwas conducted using four sterilizing times (i.e., 30, 45, 60 and 90minutes).

[0161] At the end of the ozone treatment period, the flush phase of thecycle is initiated using a vacuum pump to evacuate the chamber. Thechamber can then be opened and the samples removed.

[0162] The operating procedure and cycle events described above weremodified in some experiments to investigate the effects of pressuringthe chamber during ozone treatment. This is referred to as the“pressure” mode. The difference from “continuous flow” mode was thatimmediately at the start of the sterilizing phase, ozone was divertedinto the sterilization chamber but the chamber outlet was closed tocreate a back pressure. This meant that once the vacuum inside thechamber was broken, the pressure inside the chamber built up. This meantthat the samples were subjected to an increased ozone gas pressureduring the sterilization phase of the cycle. The chamber was flushed inthe normal way.

[0163] Static ozone gas treatment was investigated by subjecting samplesto a single chamber filling of ozone. This is referred to as the “singlefill” mode. This was accomplished by modifying the sterilizing phase sothat once vacuum was broken and the ozone concentration in the chamberwas 10 wt %, both the inlet and outlet valves were closed. This sealsthe ozone in the chamber with no further gas inflow, hence the samplesare maintained in a static ozone atmosphere for the remainder of thesterilization phase. The flush phase was accomplished in the usual way.

[0164] Paper spore strips (Duo-Spore, Propper Manufacturing Co., Inc.,Long Island City N.Y.) containing Bacillus stearothermophilus andBacillus subtilis var. niger were used for all tests. Spore strips weretested both with and without glassine envelopes. The strips were placedon a metal rack at a height mid-way between the top and the bottom ofthe chamber. A minimum of four spore strips were placed in the chamberfor each run. For each run, two non-ozone treated spore strips were usedas controls. They were asceptically removed from their envelope andplaced immediately in sterile nutrient broth (NB). All control stripsshowed vigorous growth in NB within 24 hours.

[0165] Following ozone treatment, spore strips were asceptically removedfrom the chamber and placed in individual test tubes containing 9 ml ofsterile NB. The strips were incubated at 37° C. for 21 days. If nogrowth was visible after 21 days, the strips were heat shocked. Heatshocking was accomplished by placing the tubes containing the stripsinto an 80° C. water bath for 20 minutes. The tubes were then returnedto the incubator for an additional 14 days. Any strips not showinggrowth at the end of this time were classified as “sterile”.

[0166]FIGS. 11 & 12 illustrate the results obtained from over 80evaluations of the sporicidal activity of ozone under different chamberconditions and exposure times. A minimum of 4 replicate spore stripswere used for each evaluation. Typically each spore strip contained inexcess of 10⁶ spores (e.g., 6.0×10⁵ spores of Bacillusstearothertnophilus and 5.0×10⁶ spores of Bacillus subtilis var. niger).The results of these evaluations are expressed in terms of “% of sporestrips sterilized” for each exposure time period and each operationalmode. Overkill was achieved when 100% of the spore strips weresterilized.

[0167]FIG. 11 contains the results of tests conducted on spore stripsenclosed in glassine envelopes. FIG. 12 shows the results of testsconducted on spore strips which had been removed from the glassineenvelopes. These data clearly show that the glassine envelopes (FIG. 11)had a small but observable negative effect on the sporicidal activity ofozone. The glassine envelopes represented a physical barrier whichdelayed the contact and penetration of ozone into the spores, thusdecreasing the overall sporicidal efficiency of the ozone.

[0168] The results seen in FIGS. 11 & 12 clearly demonstrate that the“pressure” mode of operation was the most effective. An increase ofchamber pressure significantly enhanced sporicidal activity, andoverkill was achieved after a 45 minute exposure. The “flow through”scheme was the next most effective mode of operation. This operationalmode displayed a significantly lower level of spore inactivation at the30 and 45 minute exposure times than did the “pressure” mode. With thisoperational mode, overkill was achieved after a 60 minute exposure. The“single fill” was the least effective operational mode. A 90 minuteozone exposure was required to achieve overkill.

[0169]FIG. 13 shows the results of experiments on the sterilizingeffects of ozone at 1, 3 and 9 weight percent (wt %) using BiologicalIndicators (BIs). The biological indicator was spores of the bacteriumBacillus subtilis var niger on ceramic penicylinder carriers. The BIswere mixed with a medical waste load, which was continuously agitated.The results are expressed as the fraction of negative BIs (i.e., thefraction of treated BIs that showed no growth after 14 days). The graphshows that the time required for overkill (total inactivation of spores)depended on the ozone concentration. Increasing the ozone concentrationdecreases the time required for overkill. Overkill was achieved after 30minutes at 9 wt %, 60 minutes for 3 wt % and between 90 and 120 minutesfor 1 wt % ozone.

[0170] A D-value is defined as the amount of time required by asporicidal agent to effect a one log reduction in the number of viablespores present in a sample. The D-values given for the experimentsconducted during this work were extrapolated from the overkill timerequirements. The extrapolation was based on the followingassumptions: 1) individual spore strips contained in excess of 10⁶spores; and 2) the time required to achieve overkill (100%) representeda six log reduction. Based on these assumptions, the “pressure” modewith an overkill time requirement of 45 minutes yielded an extrapolatedD-value of 7.5 minutes. The “flow through” mode achieved overkill at 60minutes, this yielded an extrapolated D-value of 10 minutes. While the“single fill” mode required 90 minutes to achieve overkill, resulting inan extrapolated D-value of 15 minutes.

[0171] A sterilization cycle time of approximately one hour was chosento make this technology competitive with steam and hydrogen peroxidesterilization systems. Therefore, the ozone production rate and theinstrument sterilization volume was sized to match the 1 hoursterilization period.

[0172] One preferred sterilization system has the ability to sterilizeboth 18″×6″×3″ nalgene and 12.5″×8.25″×2.5″ stainless steel standardsterilization trays. This, in effect sets both the sterilization chambercolumn and dimensions which are 11 liters of 20″×9.75″×3.5″respectively. Furthermore, as a baseline, an electrochemical cell stackcapable of producing a total gas flow rate of 2.2 lpm was selected. Witha sterilization chamber volume of 11 liters and a gas flow rate of 2.2lpm, it will take approximately five minutes to fill the chamber(provided the ambient air initially occupying the chamber is firstevacuated). An ozone exposure time of 60 minutes is all the timerequired to sterilize commercial spore strips, leaving five minutes topurge the chamber after the sterilization period has elapsed. Thus, asterilization cycle time of just over an hour was achieved.

[0173] The above operating sequence is the core design rationale fordevelopment and operation of the ozone sterilizer prototype. Howeverthis design rationale does not address the continuous operatingcharacteristic of the cell stack. Only small amounts of ozone arerequired to maintain the concentration in the sterilization chamber. Butthe cell stack is continuing to generate the ozone/oxygen mixture at atotal gas flow rate of 2.2 lpm and power is being consumed during thistime. Additionally, ozone continues to be generated during preparationand retrieval of the instruments from the sterilization chamber.

[0174] To fully utilize the continuous ozone generating characteristicof electrochemical cells, multiple sterilization chambers may be used.Furthermore, the sterilization chambers may be capable of being removedfrom the main sterilization system in order to allow new chambers to besterilized. These portable sterilization chambers or “cassettes” workingin conjunction with an accumulator provides a highly effective way tofully utilize the continuous operating nature of the cell stack. Theaccumulator serves as a temporary storage place for the ozone/oxygen gasmixture while the cassettes are being changed out, loaded withinstruments or delivered to the operating room. The ozone fluiddistribution system is designed so that the accumulator is continuouslyreplenished with fresh ozone thus maintaining high ozone concentrationfor “on-demand” filling of the cassettes. The ozone distribution iscontrolled by a series of electrically actuated solenoid valves whichare energized at the appropriate time by the system's on-boardmicrocontroller. For the prototype, a three cassette design wasimplemented which most closely matches the cell stack ozone output. Aside benefit of the portable cassette approach is the reduction ofsecondary contamination during transport of instruments to the operatingroom. An adaptation of these methods can be used to sterilize endoscopesby flowing ozone gas through each channel.

[0175] Disinfection of medical wastes can also be effected using ozone.It is preferred that the medical waste be shredded and placed into achamber where the shredded or particulate waste can be agitated.Intimate contact between the waste and ozone is achieved as ozone ispassed through the chamber. The moisture content is varied by adding asaline solution to the waste mixture. Ozone concentration and pressurewill improve the disinfection process in the same manner as describedabove.

Destruction of Aflatoxins in Grain

[0176] Mycotoxins are naturally occurring chemical compounds that areproduced by certain species of fungi (e.g., Aspergillus, Fusarium,Penicillium) which grow on organic materials such as grains. Mycotoxinsinclude aflatoxins, fumonosin, cyclopiezoic acid, ochratoxin, patulin,secalonic acid A, and zearalenone. They are most often produced in thefield under conditions of environmental stress (heat and drought).Aflatoxins (AFs) are the most commonly occurring and best known of themycotoxins, and are also among the most potent. While the major forms,Aflatoxin B₁ and Aflatoxin G₁, are particularly noted as carcinogens(primarily for the liver), all of the aflatoxins can have adverseeffects on virtually all other organ systems. These toxic effects can beeither acute or chronic, depending on the level and duration of AFexposure and the species. Virtually all animals in the food chain can beaffected by consumption of contaminated grain including humans, who canbe exposed directly through grain handling and consumption or indirectlythrough consumption of contaminated livestock or livestock products(e.g., milk). As a result, aflatoxin contamination of grains such ascorn and peanuts can create severe economic losses at all levels of foodproduction, e.g., pre-harvest prevention, post-harvest treatment,down-grading or outright loss of contaminated grain, decreasedproductivity and increased loss of livestock, health care costs, etc.

[0177] Methods for detoxification of contaminated grain fall into threebasic categories: physical, chemical or biological. Aflatoxins aregenerally resistant to physical methods of destruction (e.g., heat,irradiation), while mechanical- and hand-sorting of infected kernels istime-consuming and expensive.

[0178] The inventor's research has shown that ozonation has promise asan effective means of aflatoxin destruction in solution where ozone isprovided in high concentrations and is a more effective reagent than dryO₃ gas. Preliminary evidence suggests that this method of gaseous ozoneproduction can be applied effectively for the rapid destruction ofaflatoxins. Ozone appears to immediately attack the C₈₋₉ double bond inaflatoxin B₁ and aflatoxin G₁ by electrophillic addition, while moreslowly cleaving the coumarin moiety of aflatoxin B₂ and aflatoxin G₂. Inaddition, ozone is an effective sterilant and thus may reducepost-treatment growth of microorganisms during storage. It also may becapable of destroying pesticide residues (e.g., Captan, Dexon) and otherundesirable contaminants.

[0179] It is anticipated that in addition to reducing aflatoxincontamination, this methodology may be useful for destruction of othermycotoxins (e.g., patulin, T₂ toxin, zearelenone) as well as acting as asterilant to reduce post-treatment microbial growth. It is also believedthat no new toxic compounds are formed and that the nutritive value ofthe feed is not seriously affected by ozone treatment.

[0180] Two preferred systems are shown schematically in FIGS. 14 and15.In FIG. 14, the ozone is forced under pressure through the bulk grainfor the chosen duration of treatment. If necessary, the grain may easilybe agitated (e.g., with an auger, from the top) to improve permeation ofthe O₃ throughout the sample. In a fully automated system, theappearance of residual ozone exiting the grain may be used as anindicator of completion of the treatment and used as a signal to shutdown further ozone production.

[0181] In the second system (FIG. 15), the grain is moved from onestorage bin to another with an auger. During the transit, the grain ismoved past multiple ports for the input of the O₃ and thoroughly mixedwith the gas. The auger speed can be adjusted to vary the rate of graintransfer and thus, the time of exposure of each portion of the grain tothe input ozone. It is anticipated that this method will require lesstotal contact time for effective treatment, and therefore less totalozone, than the first procedure.

[0182] In one example, approximately equal amounts of aflatoxin B₁, B₂,G₁ and G₂ were dissolved in a small amount of acetonitrile for furtherdistribution in water. Gaseous ozone was generated electrochemicallyfrom a 25 cm² electrolyzer cell at a rate of 20 mg O₃/min and bubbledinto the aflatoxin solutions for intervals of 15 seconds to 15 minutes.Upon termination of ozonation, an equal volume of chloroform was addedimmediately to the aqueous solution and the reaction products extractedby vortexing for 30 sec. The extracts were analyzed by HPLC foraflatoxin content. Aflatoxin B₁ and G₁ were totally degraded within 15sec. aflatoxin B₂ and G₂ were slightly more resistant, requiring between1-2.5 min for complete destruction.

[0183] In additional work, a standard corn-soybean meal feed mixture wascontaminated with 750 ppb total aflatoxin as described above. Whenaliquots of the feed were simply spread on a tray and exposed to gaseousO₃ in a cabinet (3% O₃, ˜50% r.h.) for up to 1 hour, no decrease in theaflatoxin content was seen. However, exposing a similar aliquot of themixture to a continuous flow-through of ozone (similar to thearrangement shown in FIG. 14) resulted in a measurable decrease of thetotal aflatoxin in as little as 5 minutes of treatment. While thevariation was high in these initial trials (not shown), they alsodemonstrated the importance of the manner in which the ozone ispresented to the grain to be treated.

Soil Treatment Process

[0184] Soil contaminated by various organic compounds (i.e., petroleumproducts, solvents) is one of today's most extensive environmentalpollution problems. In addition, thousands of hazardous waste spillsoccur per year in the USA that require clean up under emergency responseconditions. In many instances, it is standard practice to excavate andtransport the contaminated soil to a suitable landfill or to incineratethe soil to remove the contaminant. Such practices are expensive,environmentally disruptive, require extensive permitting and only movethe contamination from one location to another. On-site and in-situtreatments of contaminated soil are safe and economical methods ofpermanently solving this problem. However, processes such as soilwashing and incineration, that may be employed on site, produce solventswaste or ash that requires disposal.

[0185] Bioremediation is one of the most promising in-situ remediationmethods currently being explored. This method uses either introduced orindigenous microorganisms to destroy or reduce the concentration of oneor more organic wastes in a contaminated site. It has numerousadvantages over other remediation methods because the soil retains itsability to support plant growth. The high costs of traditional soilremediation methods (i.e., flushing, washing, excavation and disposal)are not incurred, and the contamination is destroyed, not justtransferred elsewhere. However, one of the main difficulties ofbioremediation is that many contaminants are biorefractory or are toxicto microorganisms so that the rate of contaminant removal is too slow tobe practical.

[0186] The problem of biorefractory compounds in soil can be solved byapplying a chemical oxidation step to the soil before biologicaltreatment, to partially decompose the contaminants into intermediatesthat are more readily biodegraded. The present invention includes amethod of using gaseous ozone as an agent for decomposing chemicals incontaminated soils. Ozone is an oxidant that is able to react with manyorganic chemicals. Experiments conducted with small scale soil columnsindicate that ozone can degrade a broad range of contaminants in naturalsoil (e.g. polyaromatic and chlorinated hydrocarbons). These experimentssuggest that it may be feasible to remediate soil contaminated withorganic pollutants despite the presence of natural organic carbonmatter, that exerts a significant ozone demand (i.e., consumes ozonebefore it reacts with the target pollutants). Past research has shownthat it is difficult to transport ozone in saturated soils. However,this observation should not be translated to ozone venting which is thetransport of gaseous ozone in unsaturated soils. The concentration ofozone in the gas phase is orders of magnitude higher than in aqueoussolutions and the flow rates than can be achieved are much higher.Furthermore the half life of ozone in water is 20 minutes at best,whereas ozone gas has a half life of 24 hours.

[0187] The electrochemical method of ozone generation has a number ofcritical advantages over the corona discharge process for ozone ventingapplications. These are highlighted below.

[0188] 1. High concentration. The concentration of ozone generated bythis method (15-20 wt %) is substantially higher than is obtained by theconventional method. Electrochemical ozonizers should provide enhancedrates of soil decontamination particularly in heavily contaminated soil.

[0189] 2. Self pressurization. A substantial build-up in gas pressure isachieved with no energy penalty and without the need for a compressor todeliver the ozone underground. This should enhance ozone flow in soilsof low gas permeability and will increase ozone solubility.

[0190] 3. Compact modular design. Electrochemical systems can be easilyskid or trailer mounted for field applications.

[0191] 4. Lower capital costs. Electrochemical ozonizers are projectedto cost less than the corona discharge units.

[0192] These advantages enable successful and economically viableimplementation of ozone venting. The overall aim is to develop in-situozonation, not to completely mineralize the compounds, but to produceoxidation by-products which are more bioavailable and biodegradable bythe native consortia of microorganisms. The commercial attractiveness ofin-situ ozone venting is that it is complementary to existing in-situtechniques and in some cases can be used to overcome their limitations.While bioremediation and soil vapor extraction (SVE) are leading in-situtechnologies, they both have limitations. In-situ bioremediation isoften too slow to be practical for compounds that are not readilybiodegraded. In-situ SVE is not effective for the removal ofnon-volatile and semi-volatile organics.

[0193] Ozone itself can react selectively with certain dissolvedorganics. In the case of the highly chlorinated aromatic compounds,chlorinated carboxylic acids and ketones intermediates may result. Thesecompounds are rapidly hydrolyzed in water to form carbon dioxide andHCl. Ozone decomposition in water (promoted by UV light or H₂O₂/HO₂—)leads to the formation of hydroxyl radicals.

[0194] For compounds of intermediate or low reactivity with ozonedirectly (e.g., trichloroethylene), the reaction with hydroxyl radicalsgenerated by the degradation of ozone is primarily responsible for thedegradation of the compound in soil. Only the compounds that areextremely reactive with ozone (e.g. PAHs) does the reaction withmolecular ozone appear to be the more important degradation pathway. Itwas found that the more hydrophobic PAHs (e.g. chrysene) react moreslowly than would be expected on the basis of their reactivity withozone, suggesting that partitioning of the contaminant into the soilorganic matter may reduce the reactivity of the compound. Even so, after4 hours exposure to ozone, the chrysene concentration in a contaminatedMetea soil was reduced from 100 mg/kg to 50 mg/kg. Under the sameconditions, greater than 90% removal of phenanthrene and pyrene could beachieved with an ozonation time of 1 hour.

[0195] In Ottawa sand, it was observed that in dry soil, approximately65% of the napthalene was removed using air venting for 37 hours; thisresulted in a residual napthalene concentration of 23.2 mg/kg soil. Airstripping (for 23 hours), followed by ozone-venting (for 3.2 hours)resulted in a napthalene residual of 0.65 mg/L (approx. 99.7% removal).Similar results were observed in moist soil when ozone venting wasapplied; the residual napthalene concentration obtained wasapproximately two orders of magnitude lower than that obtained whenusing air venting alone. Experiments have been conducted to study thetransport of ozone in soil columns using a number of geologicalmaterials. Ozone is readily transported through columns packed withOttawa sand. In this case, there was a rapid initial breakthrough ofozone (within 1.5 pore volumes), however, complete breakthrough was notachieved until nearly 5 pore volumes had passed through the column.

[0196] Work has also been conducted with a Metea soil. Although theozone demand exerted by the Metea soil is greater than that of theOttawa sand, breakthrough in a 10 cm column was observed inapproximately 600 pore volumes. For Borden sand, 90% ozone breakthroughwas achieved in a 30 cm column in ca. 300 pore volumes. All thegeological material studied exerted a limited (finite) ozone demand,i.e., the rate of ozone degradation in soil columns is very slow afterthe ozone demand is met. Ottawa sand exerts little ozone demand (<0.01mg O_(3/)g sand). For Metea soil the ozone demand is approximately 1.4mg O_(3/)g soil and the ozone demand for Borden sand is approximately 2mg O₃/g sand. Once the initial ozone demand is met ozone should not berapidly degraded once injected into soil.

[0197] The objective of in-situ ozonation is not to completelymineralize the compounds but to produce oxidation by-products which aremore bioavailable and biodegradable by the native consortia ofmicroorganisms. While some lower molecular weight compounds may becompletely mineralized by ozone, many of the compounds are notmineralized rapidly by ozone. It is true that ozone would kill many soilorganisms. However, there is evidence that shows that microorganisms aremobile in the subsurface. This suggests that the reinoculation ofmicroorganisms into the subsurface would be possible, certainly, insandy or other coarse grained deposits.

Wastewater Treatment

[0198] The ozone produced by the present invention may be used fornumerous applications. One example is the treatment of waste water. Suchwaste water treatment may be performed with or without the use ofultraviolet light. The ultraviolet light radiation is capable ofproducing hydroxyl radicals from ozone fed into a waste water stream.

Other Applications

[0199] The present invention further includes a method of washinglaundry without detergent. Laundry is placed into a vessel that issubstantially sealed and filled with water. A gas having an ozoneconcentration greater than about 7 weight percent into the water is thenintroduced and bubbled into the water. The ozone becomes dissolved inthe water and contacts the laundry. This contact is continued for aperiod of time sufficient to allow the ozone to clean the laundry. Theexact length of time will be effected by the ozone concentration and thedegree of contamination on the laundry. When the laundry is clean, theozone supply is shut off and the remaining ozonated water is discardedfrom the vessel. Because there may be some residual ozone on thelaundry, the laundry is allows to sit for a short period of a fewminutes so that the ozone can decompose prior to opening the vessel. Theclean laundry is then removed from the vessel for drying. Because thecleaning rate is effected by the ozone concentration, it is preferredthat the ozone be generated in an electrochemical cell.

[0200] Another application of the highly concentrated ozone productionof the present invention is in the poultry industry. Present hatchingtechnology proscribes the use of large square boxes in which eggs areincubated and hatched. These boxes become extremely contaminated withorganic waste as the chicks break through the shell. Organic materialfrom inside the egg becomes coated on the walls of the box and providesa source of bacterial growth. The concentrated ozone gas of the presentinvention would provide a means for disinfecting these boxes for reusein the hatching process.

[0201] A particular advantage of the cells according to the presentinvention is that the ozonizers may be made compact and therefore areuseful in such applications as swimming pool sanitization, control ofbio-fouling in air conditioning systems, cooling towers, industrialwaste treatment applications, i.e., such as phenol, pesticide, cyanide,dye waste, and heavy metals. Further uses include use in bottling andmaintaining potable water quality in remote sites, reprocessing aquariawater, odor control or disinfection of sewage. Many of theseapplications are not currently utilizing ozone due to the high cost ofozonizers heretofore known, the associated cost of air or oxygenpreparation and the low concentration of ozone output

[0202] It will be understood that certain combinations andsub-combinations of the invention are of utility and may be employedwithout reference to other features or sub-combinations. This iscontemplated by and is within the scope of the present invention. Manypossible embodiments may be made of this invention without departingfrom the spirit and scope thereof. It is to be understood that allmatters herein above set forth or shown in the accompanying drawings areto be interpreted as illustrative and not in any limiting sense.

[0203] While the foregoing is directed to the preferred embodiment, thescope thereof is determined by the claims which follow:

What is claimed is:
 1. A method of sterilizing instruments comprisingthe steps of: placing biologically contaminated instruments into anenclosed chamber, evacuating air from the chamber; introducing a gashaving an ozone concentration greater than about 7 weight percent intothe chamber; contacting the instruments with the gas for a period oftime sufficient to allow the ozone to sterilize contaminants on theinstruments; withdrawing the gas from the chamber at a rate sufficientto maintain an average ozone concentration in the chamber that isgreater than about 4 weight percent; removing the gas from the chamber;and removing the sterilized instruments.
 2. The method of claim 1further comprising the step of: pressurizing the gas within the chamberto a pressure greater than about 20 psi.
 3. The method of claim 1further comprising the step of: humidifying the gas in the chamber to besubstantially saturated with water.
 4. The method of claim 3 wherein theozone is generated in a humidified form by an electrochemical cellcomprising: an anode disposed in an anodic chamber comprising asubstrate and a catalyst coating, wherein the substrate is selected fromthe group consisting of porous titanium, titanium suboxides, platinum,tungsten, tantalum, hafnium and niobium, and wherein the catalystcoating is selected from the group consisting of lead dioxide,platinum-tungsten alloys or mixtures, glassy carbon and platinum; acathode disposed in a cathodic chamber; and a proton exchange membranehaving a first side in contact with the cathode and a second side incontact with the anodic catalyst layer, wherein the proton exchangemembrane is comprised of a perfluorinated sulphonic acid polymer.
 5. Themethod of claim 4 wherein the cathode is compatible with liquid phasecathodic depolarizers, wherein the cathode is selected from the groupconsisting of flow-by electrodes, flow-through electrodes, packed bedelectrodes, and fluidized bed electrodes.
 6. The method of claim 5wherein the cathode is a gas diffusion electrode comprising apolytetrafluoroethylene-bonded, semi-hydrophobic catalyst layersupported on a hydrophobic gas diffusion layer, wherein the catalystlayer is comprised of a proton exchange polymer, polytetrafluorethylenepolymer and a metal selected from the group consisting of platinum,palladium, gold, iridium, nickel and mixtures thereof, and wherein thegas diffusion layer has a carbon cloth or carbon paper fiber impregnatedwith a sintered mass derived from fine carbon powder and apolytetrafluoroethylene emulsion.
 7. The method of claim 2 wherein theozone is generated under pressure by an electrochemical cell comprising:an anode disposed in an anodic chamber comprising a substrate and acatalyst coating, wherein the substrate is selected from the groupconsisting of porous titanium, titanium suboxides, platinum, tungsten,tantalum, hafnium and niobium, and wherein the catalyst coating isselected from the group consisting of lead dioxide, platinum-tungstenalloys or mixtures, glassy carbon and platinum; a cathode disposed in acathodic chamber; and a proton exchange membrane having a first side incontact with the cathode and a second side in contact with the anodiccatalyst layer, wherein the proton exchange membrane is comprised of aperfluorinated sulphonic acid polymer.
 8. The method of claim 7 whereinthe cathode is compatible with liquid phase cathodic depolarizers,wherein the cathode is selected from the group consisting of flow-byelectrodes, flow-through electrodes, packed bed electrodes, andfluidized bed electrodes.
 9. The method of claim 8 wherein the cathodeis a gas diffusion electrode comprising apolytetrafluoroethylene-bonded, semi-hydrophobic catalyst layersupported on a hydrophobic gas diffusion layer, wherein the catalystlayer is comprised of a proton exchange polymer, polytetrafluorethylenepolymer and a metal selected from the group consisting of platinum,palladium, gold, iridium, nickel and mixtures thereof, and wherein thegas diffusion layer has a carbon cloth or carbon paper fiber impregnatedwith a sintered mass derived from fine carbon powder and apolytetrafluoroethylene emulsion.
 10. A method of treatingmicroorganisms present in food products comprising the step of:contacting the food products with ozone for a period of time sufficientto allow the ozone to kill the microorganisms.
 11. The method of claim10 wherein the food product is selected from the group consisting ofmeats, seafood, fish, and grains.
 12. The method of claim 11 wherein thefood product is grain and the microorganism is a fungi that producesmycotoxin.
 13. The method of claim 12 wherein the mycotoxin is selectedfrom the group consisting of aflatoxins, fumonosin, cyclopiezoic acid,ochratoxin, patulin, secalonic acid A, and zearalenone.
 14. A method oftreating microorganisms present in food products comprising the stepsof: contacting the food products with ozone for a period of timesufficient to allow the ozone to destroy toxins on the food products.15. A method of remediating contaminants in soil comprising the stepsof: preparing a well which passes substantially below an area ofcontaminated soil; introducing a gas having an ozone concentrationgreater than about 7 weight percent through the well and into thecontaminated soil; venting the gas upward through the contaminated soilfor a period of time sufficient to allow the ozone to break down thecontaminants into a form more readily digested by microbes.
 16. Themethod of claim 15 further comprising the steps of: replenishing thepopulation of microbes in the soil.
 17. A method for electrochemicalproduction of ozone comprising the steps of: supplying a source ofoxygen gas to a gas diffusion cathode, wherein the gas diffusion cathodeincludes a gas diffusion layer and a catalyst layer, said gas diffusionlayer comprising carbon cloth or carbon paper fiber impregnated with asintered mass derived from fine carbon powder and apolytetrafluoroethylene emulsion, said catalyst layer comprising aproton exchange polymer, polytetrafluorethylene polymer and a metalselected from the group consisting of platinum, palladium, gold,iridium, nickel and mixtures thereof; supplying water to an anodecomprising a substrate and a catalyst coating, wherein the substrate isselected from the group consisting of porous titanium, titaniumsuboxides, platinum, tungsten, tantalum, hafnium and niobium, andwherein the catalyst coating is selected from the group consisting oflead dioxide, platinum-tungsten alloys or mixtures, glassy carbon andplatinum; and passing an electric current through the anode and the gasdiffusion cathode separated by a proton exchange membrane comprising aperfluorinated sulfonic acid polymer material, wherein the protonexchange membrane is bonded to the catalyst layer of the gas diffusioncathode, and wherein ozone is formed at the anode and hydrogen peroxideis formed at the cathode.
 18. The method of claim 17 wherein the oxygenis supplied as air.
 19. The method of claim 17 further comprising thesteps of: combining the ozone and hydrogen peroxide into a singleproduct; adding the product into wastewater containing organicsubstances; reacting the product with the organic substances; exposingthe product-containing wastewater to ultraviolet radiation; maintaininga residual of the product in the wastewater stream; and eliminating theproduct from the wastewater before use.
 20. A method of sterilizing amixture of medical waste comprising the steps of: shredding a mixture ofmedical wastes; agitating the shredded waste in a chamber; introducing agas having an ozone concentration greater than about 7 weight percentinto the chamber; contacting the shredded waste with the gas for aperiod of time sufficient to allow the ozone to sterilize; and disposingthe sterilized medical waste.
 21. The method of claim 20 wherein theozone is generated in an electrochemical cell.
 22. A method of washinglaundry without detergent comprising the steps of: agitating laundry inan enclosed, substantially sealed vessel filled with water; introducingand bubbling a gas having an ozone concentration greater than about 7weight percent into the water; contacting the laundry with the ozonatedwater for a period of time sufficient to allow the ozone to clean thelaundry; discarding the ozonated water from the vessel; allowing theresidual ozone on the laundry to decompose; and removing the cleanlaundry from the vessel.
 23. The method of claim 22 wherein the ozone isgenerated in an electrochemical cell.